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SQLAlchemy 0.8 Documentation
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Release: <span class="version-num">0.8.7</span> | Release Date: July 22, 2014
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<h1>SQLAlchemy 0.8 Documentation</h1>
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Glossary
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<li><a class="reference internal" href="#">Glossary</a></li>
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<div class="section" id="glossary">
<span id="id1"></span><h1>Glossary<a class="headerlink" href="#glossary" title="Permalink to this headline">¶</a></h1>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The Glossary is a brand new addition to the documentation. While
sparse at the moment we hope to fill it up with plenty of new
terms soon!</p>
</div>
<dl class="glossary docutils">
<dt id="term-acid"><span id="term-acid-model"></span>ACID<br />ACID model</dt>
<dd><p class="first">An acronym for “Atomicity, Consistency, Isolation,
Durability”; a set of properties that guarantee that
database transactions are processed reliably.
(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-atomicity"><em class="xref std std-term">atomicity</em></a></p>
<p><a class="reference internal" href="#term-consistency"><em class="xref std std-term">consistency</em></a></p>
<p><a class="reference internal" href="#term-isolation"><em class="xref std std-term">isolation</em></a></p>
<p><a class="reference internal" href="#term-durability"><em class="xref std std-term">durability</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/ACID_Model">http://en.wikipedia.org/wiki/ACID_Model</a></p>
</div>
</dd>
<dt id="term-annotations">annotations</dt>
<dd><p class="first">Annotations are a concept used internally by SQLAlchemy in order to store
additional information along with <a class="reference internal" href="core/sqlelement.html#sqlalchemy.sql.expression.ClauseElement" title="sqlalchemy.sql.expression.ClauseElement"><tt class="xref py py-class docutils literal"><span class="pre">ClauseElement</span></tt></a> objects. A Python
dictionary is associated with a copy of the object, which contains key/value
pairs significant to various internal systems, mostly within the ORM:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">some_column</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'some_column'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">)</span>
<span class="n">some_column_annotated</span> <span class="o">=</span> <span class="n">some_column</span><span class="o">.</span><span class="n">_annotate</span><span class="p">({</span><span class="s">"entity"</span><span class="p">:</span> <span class="n">User</span><span class="p">})</span></pre></div>
</div>
<p class="last">The annotation system differs from the public dictionary <a class="reference internal" href="core/metadata.html#sqlalchemy.schema.Column.info" title="sqlalchemy.schema.Column.info"><tt class="xref py py-attr docutils literal"><span class="pre">Column.info</span></tt></a>
in that the above annotation operation creates a <em>copy</em> of the new <a class="reference internal" href="core/metadata.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt></a>,
rather than considering all annotation values to be part of a single
unit. The ORM creates copies of expression objects in order to
apply annotations that are specific to their context, such as to differentiate
columns that should render themselves as relative to a joined-inheritance
entity versus those which should render relative to their immediate parent
table alone, as well as to differentiate columns within the “join condition”
of a relationship where the column in some cases needs to be expressed
in terms of one particular table alias or another, based on its position
within the join expression.</p>
</dd>
<dt id="term-association-relationship">association relationship</dt>
<dd><p class="first">A two-tiered <a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a> which links two tables
together using an association table in the middle. The
association relationship differs from a <a class="reference internal" href="#term-many-to-many"><em class="xref std std-term">many to many</em></a>
relationship in that the many-to-many table is mapped
by a full class, rather than invisibly handled by the
<a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">sqlalchemy.orm.relationship()</span></tt></a> construct as in the case
with many-to-many, so that additional attributes are
explicitly available.</p>
<p>For example, if we wanted to associate employees with
projects, also storing the specific role for that employee
with the project, the relational schema might look like:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">project</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee_project</span> <span class="p">(</span>
<span class="n">employee_id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">project_id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">role_name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">),</span>
<span class="k">FOREIGN</span> <span class="k">KEY</span> <span class="n">employee_id</span> <span class="k">REFERENCES</span> <span class="n">employee</span><span class="p">(</span><span class="n">id</span><span class="p">),</span>
<span class="k">FOREIGN</span> <span class="k">KEY</span> <span class="n">project_id</span> <span class="k">REFERENCES</span> <span class="n">project</span><span class="p">(</span><span class="n">id</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>A SQLAlchemy declarative mapping for the above might look like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Employee</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">Project</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'project'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">EmployeeProject</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee_project'</span>
<span class="n">employee_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'employee.id'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">project_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'project.id'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">role_name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">project</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Project"</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s">"project_employees"</span><span class="p">)</span>
<span class="n">employee</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Employee"</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s">"employee_projects"</span><span class="p">)</span></pre></div>
</div>
<p>Employees can be added to a project given a role name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">proj</span> <span class="o">=</span> <span class="n">Project</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"Client A"</span><span class="p">)</span>
<span class="n">emp1</span> <span class="o">=</span> <span class="n">Employee</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"emp1"</span><span class="p">)</span>
<span class="n">emp2</span> <span class="o">=</span> <span class="n">Employee</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"emp2"</span><span class="p">)</span>
<span class="n">proj</span><span class="o">.</span><span class="n">project_employees</span><span class="o">.</span><span class="n">extend</span><span class="p">([</span>
<span class="n">EmployeeProject</span><span class="p">(</span><span class="n">employee</span><span class="o">=</span><span class="n">emp1</span><span class="p">,</span> <span class="n">role</span><span class="o">=</span><span class="s">"tech lead"</span><span class="p">),</span>
<span class="n">EmployeeProject</span><span class="p">(</span><span class="n">employee</span><span class="o">=</span><span class="n">emp2</span><span class="p">,</span> <span class="n">role</span><span class="o">=</span><span class="s">"account executive"</span><span class="p">)</span>
<span class="p">])</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="#term-many-to-many"><em class="xref std std-term">many to many</em></a></p>
</div>
</dd>
<dt id="term-atomicity">atomicity</dt>
<dd><p class="first">Atomicity is one of the components of the <a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a> model,
and requires that each transaction is “all or nothing”:
if one part of the transaction fails, the entire transaction
fails, and the database state is left unchanged. An atomic
system must guarantee atomicity in each and every situation,
including power failures, errors, and crashes.
(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Atomicity_(database_systems">http://en.wikipedia.org/wiki/Atomicity_(database_systems</a>)</p>
</div>
</dd>
<dt id="term-backref"><span id="term-bidirectional-relationship"></span>backref<br />bidirectional relationship</dt>
<dd><p class="first">An extension to the <a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a> system whereby two
distinct <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> objects can be
mutually associated with each other, such that they coordinate
in memory as changes occur to either side. The most common
way these two relationships are constructed is by using
the <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> function explicitly
for one side and specifying the <tt class="docutils literal"><span class="pre">backref</span></tt> keyword to it so that
the other <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> is created
automatically. We can illustrate this against the example we’ve
used in <a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a> as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Department</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'department'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">employees</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Employee"</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s">"department"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Employee</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">dep_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'department.id'</span><span class="p">))</span></pre></div>
</div>
<p>A backref can be applied to any relationship, including one to many,
many to one, and <a class="reference internal" href="#term-many-to-many"><em class="xref std std-term">many to many</em></a>.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a></p>
<p><a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a></p>
<p><a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a></p>
<p class="last"><a class="reference internal" href="#term-many-to-many"><em class="xref std std-term">many to many</em></a></p>
</div>
</dd>
<dt id="term-candidate-key">candidate key</dt>
<dd><p class="first">A <em class="xref std std-term">relational algebra</em> term referring to an attribute or set
of attributes that form a uniquely identifying key for a
row. A row may have more than one candidate key, each of which
is suitable for use as the primary key of that row.
The primary key of a table is always a candidate key.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-primary-key"><em class="xref std std-term">primary key</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Candidate_key">http://en.wikipedia.org/wiki/Candidate_key</a></p>
</div>
</dd>
<dt id="term-check-constraint">check constraint</dt>
<dd><p class="first">A check constraint is a
condition that defines valid data when adding or updating an
entry in a table of a relational database. A check constraint
is applied to each row in the table.</p>
<p>(via Wikipedia)</p>
<p>A check constraint can be added to a table in standard
SQL using <a class="reference internal" href="#term-ddl"><em class="xref std std-term">DDL</em></a> like the following:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">ALTER</span> <span class="k">TABLE</span> <span class="n">distributors</span> <span class="k">ADD</span> <span class="k">CONSTRAINT</span> <span class="n">zipchk</span> <span class="k">CHECK</span> <span class="p">(</span><span class="k">char_length</span><span class="p">(</span><span class="n">zipcode</span><span class="p">)</span> <span class="o">=</span> <span class="mi">5</span><span class="p">);</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Check_constraint">http://en.wikipedia.org/wiki/Check_constraint</a></p>
</div>
</dd>
<dt id="term-columns-clause">columns clause</dt>
<dd><p class="first">The portion of the <tt class="docutils literal"><span class="pre">SELECT</span></tt> statement which enumerates the
SQL expressions to be returned in the result set. The expressions
follow the <tt class="docutils literal"><span class="pre">SELECT</span></tt> keyword directly and are a comma-separated
list of individual expressions.</p>
<p>E.g.:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">user_account</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">user_account</span><span class="p">.</span><span class="n">email</span>
<span class="k">FROM</span> <span class="n">user_account</span> <span class="k">WHERE</span> <span class="n">user_account</span><span class="p">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'fred'</span></pre></div>
</div>
<p class="last">Above, the list of columns <tt class="docutils literal"><span class="pre">user_acount.name</span></tt>,
<tt class="docutils literal"><span class="pre">user_account.email</span></tt> is the columns clause of the <tt class="docutils literal"><span class="pre">SELECT</span></tt>.</p>
</dd>
<dt id="term-consistency">consistency</dt>
<dd><p class="first">Consistency is one of the compoments of the <a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a> model,
and ensures that any transaction will
bring the database from one valid state to another. Any data
written to the database must be valid according to all defined
rules, including but not limited to <a class="reference internal" href="#term-constraints"><em class="xref std std-term">constraints</em></a>, cascades,
triggers, and any combination thereof.
(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Consistency_(database_systems">http://en.wikipedia.org/wiki/Consistency_(database_systems</a>)</p>
</div>
</dd>
<dt id="term-constraint"><span id="term-constraints"></span><span id="term-constrained"></span>constraint<br />constraints<br />constrained</dt>
<dd>Rules established within a relational database that ensure
the validity and consistency of data. Common forms
of constraint include <a class="reference internal" href="#term-primary-key-constraint"><em class="xref std std-term">primary key constraint</em></a>,
<a class="reference internal" href="#term-foreign-key-constraint"><em class="xref std std-term">foreign key constraint</em></a>, and <a class="reference internal" href="#term-check-constraint"><em class="xref std std-term">check constraint</em></a>.</dd>
<dt id="term-correlates"><span id="term-correlated-subquery"></span><span id="term-correlated-subqueries"></span>correlates<br />correlated subquery<br />correlated subqueries</dt>
<dd><p class="first">A <a class="reference internal" href="#term-subquery"><em class="xref std std-term">subquery</em></a> is correlated if it depends on data in the
enclosing <tt class="docutils literal"><span class="pre">SELECT</span></tt>.</p>
<p>Below, a subquery selects the aggregate value <tt class="docutils literal"><span class="pre">MIN(a.id)</span></tt>
from the <tt class="docutils literal"><span class="pre">email_address</span></tt> table, such that
it will be invoked for each value of <tt class="docutils literal"><span class="pre">user_account.id</span></tt>, correlating
the value of this column against the <tt class="docutils literal"><span class="pre">email_address.user_account_id</span></tt>
column:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">user_account</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">email_address</span><span class="p">.</span><span class="n">email</span>
<span class="k">FROM</span> <span class="n">user_account</span>
<span class="k">JOIN</span> <span class="n">email_address</span> <span class="k">ON</span> <span class="n">user_account</span><span class="p">.</span><span class="n">id</span><span class="o">=</span><span class="n">email_address</span><span class="p">.</span><span class="n">user_account_id</span>
<span class="k">WHERE</span> <span class="n">email_address</span><span class="p">.</span><span class="n">id</span> <span class="o">=</span> <span class="p">(</span>
<span class="k">SELECT</span> <span class="k">MIN</span><span class="p">(</span><span class="n">a</span><span class="p">.</span><span class="n">id</span><span class="p">)</span> <span class="k">FROM</span> <span class="n">email_address</span> <span class="k">AS</span> <span class="n">a</span>
<span class="k">WHERE</span> <span class="n">a</span><span class="p">.</span><span class="n">user_account_id</span><span class="o">=</span><span class="n">user_account</span><span class="p">.</span><span class="n">id</span>
<span class="p">)</span></pre></div>
</div>
<p>The above subquery refers to the <tt class="docutils literal"><span class="pre">user_account</span></tt> table, which is not itself
in the <tt class="docutils literal"><span class="pre">FROM</span></tt> clause of this nested query. Instead, the <tt class="docutils literal"><span class="pre">user_account</span></tt>
table is received from the enclosing query, where each row selected from
<tt class="docutils literal"><span class="pre">user_account</span></tt> results in a distinct execution of the subquery.</p>
<p>A correlated subquery is in most cases present in the <a class="reference internal" href="#term-where-clause"><em class="xref std std-term">WHERE clause</em></a>
or <a class="reference internal" href="#term-columns-clause"><em class="xref std std-term">columns clause</em></a> of the immediately enclosing <tt class="docutils literal"><span class="pre">SELECT</span></tt>
statement, as well as in the ORDER BY or HAVING clause.</p>
<p>In less common cases, a correlated subquery may be present in the
<a class="reference internal" href="#term-from-clause"><em class="xref std std-term">FROM clause</em></a> of an enclosing <tt class="docutils literal"><span class="pre">SELECT</span></tt>; in these cases the
correlation is typically due to the enclosing <tt class="docutils literal"><span class="pre">SELECT</span></tt> itself being
enclosed in the WHERE,
ORDER BY, columns or HAVING clause of another <tt class="docutils literal"><span class="pre">SELECT</span></tt>, such as:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">parent</span><span class="p">.</span><span class="n">id</span> <span class="k">FROM</span> <span class="n">parent</span>
<span class="k">WHERE</span> <span class="k">EXISTS</span> <span class="p">(</span>
<span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="p">(</span>
<span class="k">SELECT</span> <span class="n">child</span><span class="p">.</span><span class="n">id</span> <span class="k">AS</span> <span class="n">id</span><span class="p">,</span> <span class="n">child</span><span class="p">.</span><span class="n">parent_id</span> <span class="k">AS</span> <span class="n">parent_id</span><span class="p">,</span> <span class="n">child</span><span class="p">.</span><span class="n">pos</span> <span class="k">AS</span> <span class="n">pos</span>
<span class="k">FROM</span> <span class="n">child</span>
<span class="k">WHERE</span> <span class="n">child</span><span class="p">.</span><span class="n">parent_id</span> <span class="o">=</span> <span class="n">parent</span><span class="p">.</span><span class="n">id</span> <span class="k">ORDER</span> <span class="k">BY</span> <span class="n">child</span><span class="p">.</span><span class="n">pos</span>
<span class="k">LIMIT</span> <span class="mi">3</span><span class="p">)</span>
<span class="k">WHERE</span> <span class="n">id</span> <span class="o">=</span> <span class="mi">7</span><span class="p">)</span></pre></div>
</div>
<p class="last">Correlation from one <tt class="docutils literal"><span class="pre">SELECT</span></tt> directly to one which encloses the correlated
query via its <tt class="docutils literal"><span class="pre">FROM</span></tt>
clause is not possible, because the correlation can only proceed once the
original source rows from the enclosing statement’s FROM clause are available.</p>
</dd>
<dt id="term-crud">crud</dt>
<dd>An acronym meaning “Create, Update, Delete”. The term in SQL refers to the
set of operations that create, modify and delete data from the database,
also known as <em class="xref std std-term">DML</em>, and typically refers to the <tt class="docutils literal"><span class="pre">INSERT</span></tt>,
<tt class="docutils literal"><span class="pre">UPDATE</span></tt>, and <tt class="docutils literal"><span class="pre">DELETE</span></tt> statements.</dd>
<dt id="term-dbapi">DBAPI</dt>
<dd><p class="first">DBAPI is shorthand for the phrase “Python Database API
Specification”. This is a widely used specification
within Python to define common usage patterns for all
database connection packages. The DBAPI is a “low level”
API which is typically the lowest level system used
in a Python application to talk to a database. SQLAlchemy’s
<em class="xref std std-term">dialect</em> system is constructed around the
operation of the DBAPI, providing individual dialect
classes which service a specific DBAPI on top of a
specific database engine; for example, the <a class="reference internal" href="core/engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a>
URL <tt class="docutils literal"><span class="pre">postgresql+psycopg2://@localhost/test</span></tt>
refers to the <a class="reference internal" href="dialects/postgresql.html#module-sqlalchemy.dialects.postgresql.psycopg2" title="sqlalchemy.dialects.postgresql.psycopg2"><tt class="xref py py-mod docutils literal"><span class="pre">psycopg2</span></tt></a>
DBAPI/dialect combination, whereas the URL <tt class="docutils literal"><span class="pre">mysql+mysqldb://@localhost/test</span></tt>
refers to the <a class="reference internal" href="dialects/mysql.html#module-sqlalchemy.dialects.mysql.mysqldb" title="sqlalchemy.dialects.mysql.mysqldb"><tt class="xref py py-mod docutils literal"><span class="pre">MySQL</span> <span class="pre">for</span> <span class="pre">Python</span></tt></a>
DBAPI DBAPI/dialect combination.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://www.python.org/dev/peps/pep-0249/">PEP 249 - Python Database API Specification v2.0</a></p>
</div>
</dd>
<dt id="term-ddl">DDL</dt>
<dd><p class="first">An acronym for <em>Data Definition Language</em>. DDL is the subset
of SQL that relational databases use to configure tables, constraints,
and other permanent objects within a database schema. SQLAlchemy
provides a rich API for constructing and emitting DDL expressions.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="core/metadata.html"><em>Describing Databases with MetaData</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Data_definition_language">DDL (via Wikipedia)</a></p>
</div>
</dd>
<dt id="term-descriptor"><span id="term-descriptors"></span>descriptor<br />descriptors</dt>
<dd><p class="first">In Python, a descriptor is an object attribute with “binding behavior”, one whose attribute access has been overridden by methods in the <a class="reference external" href="http://docs.python.org/howto/descriptor.html">descriptor protocol</a>.
Those methods are __get__(), __set__(), and __delete__(). If any of those methods are defined
for an object, it is said to be a descriptor.</p>
<p>In SQLAlchemy, descriptors are used heavily in order to provide attribute behavior
on mapped classes. When a class is mapped as such:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'foo'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">)</span></pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">MyClass</span></tt> class will be <a class="reference internal" href="#term-mapped"><em class="xref std std-term">mapped</em></a> when its definition
is complete, at which point the <tt class="docutils literal"><span class="pre">id</span></tt> and <tt class="docutils literal"><span class="pre">data</span></tt> attributes,
starting out as <a class="reference internal" href="core/metadata.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt></a> objects, will be replaced
by the <a class="reference internal" href="#term-instrumentation"><em class="xref std std-term">instrumentation</em></a> system with instances
of <a class="reference internal" href="orm/internals.html#sqlalchemy.orm.attributes.InstrumentedAttribute" title="sqlalchemy.orm.attributes.InstrumentedAttribute"><tt class="xref py py-class docutils literal"><span class="pre">InstrumentedAttribute</span></tt></a>, which are descriptors that
provide the above mentioned <tt class="docutils literal"><span class="pre">__get__()</span></tt>, <tt class="docutils literal"><span class="pre">__set__()</span></tt> and
<tt class="docutils literal"><span class="pre">__delete__()</span></tt> methods. The <a class="reference internal" href="orm/internals.html#sqlalchemy.orm.attributes.InstrumentedAttribute" title="sqlalchemy.orm.attributes.InstrumentedAttribute"><tt class="xref py py-class docutils literal"><span class="pre">InstrumentedAttribute</span></tt></a>
will generate a SQL expression when used at the class level:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</span> <span class="n">MyClass</span><span class="o">.</span><span class="n">data</span> <span class="o">==</span> <span class="mi">5</span>
<span class="go">data = :data_1</span></pre></div>
</div>
<p>and at the instance level, keeps track of changes to values,
and also <a class="reference internal" href="#term-lazy-loads"><em class="xref std std-term">lazy loads</em></a> unloaded attributes
from the database:</p>
<div class="last highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">m1</span> <span class="o">=</span> <span class="n">MyClass</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">m1</span><span class="o">.</span><span class="n">id</span> <span class="o">=</span> <span class="mi">5</span>
<span class="gp">>>> </span><span class="n">m1</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="s">"some data"</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">inspect</span>
<span class="gp">>>> </span><span class="n">inspect</span><span class="p">(</span><span class="n">m1</span><span class="p">)</span><span class="o">.</span><span class="n">attrs</span><span class="o">.</span><span class="n">data</span><span class="o">.</span><span class="n">history</span><span class="o">.</span><span class="n">added</span>
<span class="go">"some data"</span></pre></div>
</div>
</dd>
<dt id="term-detached">detached</dt>
<dd><p class="first">This describes one of the four major object states which
an object can have within a <a class="reference internal" href="#term-session"><em class="xref std std-term">session</em></a>; a detached object
is an object that has a database identity (i.e. a primary key)
but is not associated with any session. An object that
was previously <a class="reference internal" href="#term-persistent"><em class="xref std std-term">persistent</em></a> and was removed from its
session either because it was expunged, or the owning
session was closed, moves into the detached state.
The detached state is generally used when objects are being
moved between sessions or when being moved to/from an external
object cache.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/session.html#session-object-states"><em>Quickie Intro to Object States</em></a></p>
</div>
</dd>
<dt id="term-discriminator">discriminator</dt>
<dd><p class="first">A result-set column which is used during <a class="reference internal" href="#term-polymorphic"><em class="xref std std-term">polymorphic</em></a> loading
to determine what kind of mapped class should be applied to a particular
incoming result row. In SQLAlchemy, the classes are always part
of a hierarchy mapping using inheritance mapping.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/inheritance.html"><em>Mapping Class Inheritance Hierarchies</em></a></p>
</div>
</dd>
<dt id="term-durability">durability</dt>
<dd><p class="first">Durability is a property of the <a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a> model
which means that once a transaction has been committed,
it will remain so, even in the event of power loss, crashes,
or errors. In a relational database, for instance, once a
group of SQL statements execute, the results need to be stored
permanently (even if the database crashes immediately
thereafter).
(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Durability_(database_systems">http://en.wikipedia.org/wiki/Durability_(database_systems</a>)</p>
</div>
</dd>
<dt id="term-foreign-key-constraint">foreign key constraint</dt>
<dd><p class="first">A referential constraint between two tables. A foreign key is a field or set of fields in a
relational table that matches a <a class="reference internal" href="#term-candidate-key"><em class="xref std std-term">candidate key</em></a> of another table.
The foreign key can be used to cross-reference tables.
(via Wikipedia)</p>
<p>A foreign key constraint can be added to a table in standard
SQL using <a class="reference internal" href="#term-ddl"><em class="xref std std-term">DDL</em></a> like the following:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">ALTER</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="k">ADD</span> <span class="k">CONSTRAINT</span> <span class="n">dep_id_fk</span>
<span class="k">FOREIGN</span> <span class="k">KEY</span> <span class="p">(</span><span class="n">employee</span><span class="p">)</span> <span class="k">REFERENCES</span> <span class="n">department</span> <span class="p">(</span><span class="n">dep_id</span><span class="p">)</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Foreign_key_constraint">http://en.wikipedia.org/wiki/Foreign_key_constraint</a></p>
</div>
</dd>
<dt id="term-from-clause">FROM clause</dt>
<dd><p class="first">The portion of the <tt class="docutils literal"><span class="pre">SELECT</span></tt> statement which incicates the initial
source of rows.</p>
<p>A simple <tt class="docutils literal"><span class="pre">SELECT</span></tt> will feature one or more table names in its
FROM clause. Multiple sources are separated by a comma:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="k">user</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">address</span><span class="p">.</span><span class="n">email_address</span>
<span class="k">FROM</span> <span class="k">user</span><span class="p">,</span> <span class="n">address</span>
<span class="k">WHERE</span> <span class="k">user</span><span class="p">.</span><span class="n">id</span><span class="o">=</span><span class="n">address</span><span class="p">.</span><span class="n">user_id</span></pre></div>
</div>
<p>The FROM clause is also where explicit joins are specified. We can
rewrite the above <tt class="docutils literal"><span class="pre">SELECT</span></tt> using a single <tt class="docutils literal"><span class="pre">FROM</span></tt> element which consists
of a <tt class="docutils literal"><span class="pre">JOIN</span></tt> of the two tables:</p>
<div class="last highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="k">user</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">address</span><span class="p">.</span><span class="n">email_address</span>
<span class="k">FROM</span> <span class="k">user</span> <span class="k">JOIN</span> <span class="n">address</span> <span class="k">ON</span> <span class="k">user</span><span class="p">.</span><span class="n">id</span><span class="o">=</span><span class="n">address</span><span class="p">.</span><span class="n">user_id</span></pre></div>
</div>
</dd>
<dt id="term-generative">generative</dt>
<dd>A term that SQLAlchemy uses to refer what’s normally known
as <a class="reference internal" href="#term-method-chaining"><em class="xref std std-term">method chaining</em></a>; see that term for details.</dd>
<dt id="term-identity-map">identity map</dt>
<dd><p class="first">A mapping between Python objects and their database identities.
The identity map is a collection that’s associated with an
ORM <a class="reference internal" href="#term-session"><em class="xref std std-term">session</em></a> object, and maintains a single instance
of every database object keyed to its identity. The advantage
to this pattern is that all operations which occur for a particular
database identity are transparently coordinated onto a single
object instance. When using an identity map in conjunction with
an <a class="reference internal" href="#term-isolated"><em class="xref std std-term">isolated</em></a> transaction, having a reference
to an object that’s known to have a particular primary key can
be considered from a practical standpoint to be a
proxy to the actual database row.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last">Martin Fowler - Identity Map - <a class="reference external" href="http://martinfowler.com/eaaCatalog/identityMap.html">http://martinfowler.com/eaaCatalog/identityMap.html</a></p>
</div>
</dd>
<dt id="term-instrumentation"><span id="term-instrumented"></span>instrumentation<br />instrumented</dt>
<dd>Instrumentation refers to the process of augmenting the functionality
and attribute set of a particular class. Ideally, the
behavior of the class should remain close to a regular
class, except that additional behviors and features are
made available. The SQLAlchemy <a class="reference internal" href="#term-mapping"><em class="xref std std-term">mapping</em></a> process,
among other things, adds database-enabled <a class="reference internal" href="#term-descriptors"><em class="xref std std-term">descriptors</em></a>
to a mapped
class which each represent a particular database column
or relationship to a related class.</dd>
<dt id="term-isolation"><span id="term-isolated"></span>isolation<br />isolated</dt>
<dd><p class="first">The isolation property of the <a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a> model
ensures that the concurrent execution
of transactions results in a system state that would be
obtained if transactions were executed serially, i.e. one
after the other. Each transaction must execute in total
isolation i.e. if T1 and T2 execute concurrently then each
should remain independent of the other.
(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-acid"><em class="xref std std-term">ACID</em></a></p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Isolation_(database_systems">http://en.wikipedia.org/wiki/Isolation_(database_systems</a>)</p>
</div>
</dd>
<dt id="term-lazy-load"><span id="term-lazy-loads"></span>lazy load<br />lazy loads</dt>
<dd><p class="first">In object relational mapping, a “lazy load” refers to an
attribute that does not contain its database-side value
for some period of time, typically when the object is
first loaded. Instead, the attribute receives a
<em>memoization</em> that causes it to go out to the database
and load its data when it’s first used. Using this pattern,
the complexity and time spent within object fetches can
sometimes be reduced, in that
attributes for related tables don’t need to be addressed
immediately.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference external" href="http://martinfowler.com/eaaCatalog/lazyLoad.html">Lazy Load (on Martin Fowler)</a></p>
<p><a class="reference internal" href="#term-n-plus-one-problem"><em class="xref std std-term">N plus one problem</em></a></p>
<p class="last"><a class="reference internal" href="orm/loading.html"><em>Relationship Loading Techniques</em></a></p>
</div>
</dd>
<dt id="term-many-to-many">many to many</dt>
<dd><p class="first">A style of <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">sqlalchemy.orm.relationship()</span></tt></a> which links two tables together
via an intermediary table in the middle. Using this configuration,
any number of rows on the left side may refer to any number of
rows on the right, and vice versa.</p>
<p>A schema where employees can be associated with projects:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">project</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee_project</span> <span class="p">(</span>
<span class="n">employee_id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">project_id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="k">FOREIGN</span> <span class="k">KEY</span> <span class="n">employee_id</span> <span class="k">REFERENCES</span> <span class="n">employee</span><span class="p">(</span><span class="n">id</span><span class="p">),</span>
<span class="k">FOREIGN</span> <span class="k">KEY</span> <span class="n">project_id</span> <span class="k">REFERENCES</span> <span class="n">project</span><span class="p">(</span><span class="n">id</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>Above, the <tt class="docutils literal"><span class="pre">employee_project</span></tt> table is the many-to-many table,
which naturally forms a composite primary key consisting
of the primary key from each related table.</p>
<p>In SQLAlchemy, the <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">sqlalchemy.orm.relationship()</span></tt></a> function
can represent this style of relationship in a mostly
transparent fashion, where the many-to-many table is
specified using plain table metadata:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Employee</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">projects</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span>
<span class="s">"Project"</span><span class="p">,</span>
<span class="n">secondary</span><span class="o">=</span><span class="n">Table</span><span class="p">(</span><span class="s">'employee_project'</span><span class="p">,</span> <span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">"employee_id"</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'employee.id'</span><span class="p">),</span>
<span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">"project_id"</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'project.id'</span><span class="p">),</span>
<span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="p">),</span>
<span class="n">backref</span><span class="o">=</span><span class="s">"employees"</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">Project</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'project'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span></pre></div>
</div>
<p>Above, the <tt class="docutils literal"><span class="pre">Employee.projects</span></tt> and back-referencing <tt class="docutils literal"><span class="pre">Project.employees</span></tt>
collections are defined:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">proj</span> <span class="o">=</span> <span class="n">Project</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"Client A"</span><span class="p">)</span>
<span class="n">emp1</span> <span class="o">=</span> <span class="n">Employee</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"emp1"</span><span class="p">)</span>
<span class="n">emp2</span> <span class="o">=</span> <span class="n">Employee</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"emp2"</span><span class="p">)</span>
<span class="n">proj</span><span class="o">.</span><span class="n">employees</span><span class="o">.</span><span class="n">extend</span><span class="p">([</span><span class="n">emp1</span><span class="p">,</span> <span class="n">emp2</span><span class="p">])</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-association-relationship"><em class="xref std std-term">association relationship</em></a></p>
<p><a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a></p>
<p><a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a></p>
<p class="last"><a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a></p>
</div>
</dd>
<dt id="term-many-to-one">many to one</dt>
<dd><p class="first">A style of <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> which links
a foreign key in the parent mapper’s table to the primary
key of a related table. Each parent object can
then refer to exactly zero or one related object.</p>
<p>The related objects in turn will have an implicit or
explicit <a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a> relationship to any number
of parent objects that refer to them.</p>
<p>An example many to one schema (which, note, is identical
to the <a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a> schema):</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">department</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">),</span>
<span class="n">dep_id</span> <span class="nb">INTEGER</span> <span class="k">REFERENCES</span> <span class="n">department</span><span class="p">(</span><span class="n">id</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>The relationship from <tt class="docutils literal"><span class="pre">employee</span></tt> to <tt class="docutils literal"><span class="pre">department</span></tt> is
many to one, since many employee records can be associated with a
single department. A SQLAlchemy mapping might look like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Department</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'department'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">Employee</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">dep_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'department.id'</span><span class="p">))</span>
<span class="n">department</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Department"</span><span class="p">)</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a></p>
<p><a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a></p>
<p class="last"><a class="reference internal" href="#term-backref"><em class="xref std std-term">backref</em></a></p>
</div>
</dd>
<dt id="term-mapping"><span id="term-mapped"></span>mapping<br />mapped</dt>
<dd>We say a class is “mapped” when it has been passed through the
<a class="reference internal" href="orm/mapper_config.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">orm.mapper()</span></tt></a> function. This process associates the
class with a database table or other <em class="xref std std-term">selectable</em>
construct, so that instances of it can be persisted
using a <a class="reference internal" href="orm/session.html#sqlalchemy.orm.session.Session" title="sqlalchemy.orm.session.Session"><tt class="xref py py-class docutils literal"><span class="pre">Session</span></tt></a> as well as loaded using a
<a class="reference internal" href="orm/query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><tt class="xref py py-class docutils literal"><span class="pre">Query</span></tt></a>.</dd>
<dt id="term-method-chaining">method chaining</dt>
<dd><p class="first">An object-oriented technique whereby the state of an object
is constructed by calling methods on the object. The
object features any number of methods, each of which return
a new object (or in some cases the same object) with
additional state added to the object.</p>
<p>The two SQLAlchemy objects that make the most use of
method chaining are the <a class="reference internal" href="core/selectable.html#sqlalchemy.sql.expression.Select" title="sqlalchemy.sql.expression.Select"><tt class="xref py py-class docutils literal"><span class="pre">Select</span></tt></a>
object and the <a class="reference internal" href="orm/query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><tt class="xref py py-class docutils literal"><span class="pre">Query</span></tt></a> object.
For example, a <a class="reference internal" href="core/selectable.html#sqlalchemy.sql.expression.Select" title="sqlalchemy.sql.expression.Select"><tt class="xref py py-class docutils literal"><span class="pre">Select</span></tt></a> object can
be assigned two expressions to its WHERE clause as well
as an ORDER BY clause by calling upon the <a class="reference internal" href="core/selectable.html#sqlalchemy.sql.expression.Select.where" title="sqlalchemy.sql.expression.Select.where"><tt class="xref py py-meth docutils literal"><span class="pre">where()</span></tt></a>
and <a class="reference internal" href="core/selectable.html#sqlalchemy.sql.expression.Select.order_by" title="sqlalchemy.sql.expression.Select.order_by"><tt class="xref py py-meth docutils literal"><span class="pre">order_by()</span></tt></a> methods:</p>
<div class="highlight-python"><pre>stmt = select([user.c.name]).\
where(user.c.id > 5).\
where(user.c.name.like('e%').\
order_by(user.c.name)</pre>
</div>
<p>Each method call above returns a copy of the original
<a class="reference internal" href="core/selectable.html#sqlalchemy.sql.expression.Select" title="sqlalchemy.sql.expression.Select"><tt class="xref py py-class docutils literal"><span class="pre">Select</span></tt></a> object with additional qualifiers
added.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="#term-generative"><em class="xref std std-term">generative</em></a></p>
</div>
</dd>
<dt id="term-n-plus-one-problem">N plus one problem</dt>
<dd><p class="first">The N plus one problem is a common side effect of the
<a class="reference internal" href="#term-lazy-load"><em class="xref std std-term">lazy load</em></a> pattern, whereby an application wishes
to iterate through a related attribute or collection on
each member of a result set of objects, where that
attribute or collection is set to be loaded via the lazy
load pattern. The net result is that a SELECT statement
is emitted to load the initial result set of parent objects;
then, as the application iterates through each member,
an additional SELECT statement is emitted for each member
in order to load the related attribute or collection for
that member. The end result is that for a result set of
N parent objects, there will be N + 1 SELECT statements emitted.</p>
<p>The N plus one problem is alleviated using <em class="xref std std-term">eager loading</em>.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/loading.html"><em>Relationship Loading Techniques</em></a></p>
</div>
</dd>
<dt id="term-one-to-many">one to many</dt>
<dd><p class="first">A style of <a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> which links
the primary key of the parent mapper’s table to the foreign
key of a related table. Each unique parent object can
then refer to zero or more unique related objects.</p>
<p>The related objects in turn will have an implicit or
explicit <a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a> relationship to their parent
object.</p>
<p>An example one to many schema (which, note, is identical
to the <a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a> schema):</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">department</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="p">(</span>
<span class="n">id</span> <span class="nb">INTEGER</span> <span class="k">PRIMARY</span> <span class="k">KEY</span><span class="p">,</span>
<span class="n">name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">),</span>
<span class="n">dep_id</span> <span class="nb">INTEGER</span> <span class="k">REFERENCES</span> <span class="n">department</span><span class="p">(</span><span class="n">id</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>The relationship from <tt class="docutils literal"><span class="pre">department</span></tt> to <tt class="docutils literal"><span class="pre">employee</span></tt> is
one to many, since many employee records can be associated with a
single department. A SQLAlchemy mapping might look like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Department</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'department'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">employees</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Employee"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Employee</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'employee'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">30</span><span class="p">))</span>
<span class="n">dep_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'department.id'</span><span class="p">))</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="#term-relationship"><em class="xref std std-term">relationship</em></a></p>
<p><a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a></p>
<p class="last"><a class="reference internal" href="#term-backref"><em class="xref std std-term">backref</em></a></p>
</div>
</dd>
<dt id="term-pending">pending</dt>
<dd><p class="first">This describes one of the four major object states which
an object can have within a <a class="reference internal" href="#term-session"><em class="xref std std-term">session</em></a>; a pending object
is a new object that doesn’t have any database identity,
but has been recently associated with a session. When
the session emits a flush and the row is inserted, the
object moves to the <a class="reference internal" href="#term-persistent"><em class="xref std std-term">persistent</em></a> state.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/session.html#session-object-states"><em>Quickie Intro to Object States</em></a></p>
</div>
</dd>
<dt id="term-persistent">persistent</dt>
<dd><p class="first">This describes one of the four major object states which
an object can have within a <a class="reference internal" href="#term-session"><em class="xref std std-term">session</em></a>; a persistent object
is an object that has a database identity (i.e. a primary key)
and is currently associated with a session. Any object
that was previously <a class="reference internal" href="#term-pending"><em class="xref std std-term">pending</em></a> and has now been inserted
is in the persistent state, as is any object that’s
been loaded by the session from the database. When a
persistent object is removed from a session, it is known
as <a class="reference internal" href="#term-detached"><em class="xref std std-term">detached</em></a>.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/session.html#session-object-states"><em>Quickie Intro to Object States</em></a></p>
</div>
</dd>
<dt id="term-polymorphic"><span id="term-polymorphically"></span>polymorphic<br />polymorphically</dt>
<dd><p class="first">Refers to a function that handles several types at once. In SQLAlchemy,
the term is usually applied to the concept of an ORM mapped class
whereby a query operation will return different subclasses
based on information in the result set, typically by checking the
value of a particular column in the result known as the <a class="reference internal" href="#term-discriminator"><em class="xref std std-term">discriminator</em></a>.</p>
<p class="last">Polymorphic loading in SQLAlchemy implies that a one or a
combination of three different schemes are used to map a hierarchy
of classes; “joined”, “single”, and “concrete”. The section
<a class="reference internal" href="orm/inheritance.html"><em>Mapping Class Inheritance Hierarchies</em></a> describes inheritance mapping fully.</p>
</dd>
<dt id="term-primary-key"><span id="term-primary-key-constraint"></span>primary key<br />primary key constraint</dt>
<dd><p class="first">A <a class="reference internal" href="#term-constraint"><em class="xref std std-term">constraint</em></a> that uniquely defines the characteristics
of each <em class="xref std std-term">row</em>. The primary key has to consist of
characteristics that cannot be duplicated by any other row.
The primary key may consist of a single attribute or
multiple attributes in combination.
(via Wikipedia)</p>
<p>The primary key of a table is typically, though not always,
defined within the <tt class="docutils literal"><span class="pre">CREATE</span> <span class="pre">TABLE</span></tt> <a class="reference internal" href="#term-ddl"><em class="xref std std-term">DDL</em></a>:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">employee</span> <span class="p">(</span>
<span class="n">emp_id</span> <span class="nb">INTEGER</span><span class="p">,</span>
<span class="n">emp_name</span> <span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">30</span><span class="p">),</span>
<span class="n">dep_id</span> <span class="nb">INTEGER</span><span class="p">,</span>
<span class="k">PRIMARY</span> <span class="k">KEY</span> <span class="p">(</span><span class="n">emp_id</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Primary_Key">http://en.wikipedia.org/wiki/Primary_Key</a></p>
</div>
</dd>
<dt id="term-relationship"><span id="term-relationships"></span>relationship<br />relationships</dt>
<dd><p class="first">A connecting unit between two mapped classes, corresponding
to some relationship between the two tables in the database.</p>
<p>The relationship is defined using the SQLAlchemy function
<a class="reference internal" href="orm/relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a>. Once created, SQLAlchemy
inspects the arguments and underlying mappings involved
in order to classify the relationship as one of three types:
<a class="reference internal" href="#term-one-to-many"><em class="xref std std-term">one to many</em></a>, <a class="reference internal" href="#term-many-to-one"><em class="xref std std-term">many to one</em></a>, or <a class="reference internal" href="#term-many-to-many"><em class="xref std std-term">many to many</em></a>.
With this classification, the relationship construct
handles the task of persisting the appropriate linkages
in the database in response to in-memory object associations,
as well as the job of loading object references and collections
into memory based on the current linkages in the
database.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/relationships.html"><em>Relationship Configuration</em></a></p>
</div>
</dd>
<dt id="term-release"><span id="term-releases"></span><span id="term-released"></span>release<br />releases<br />released</dt>
<dd><p class="first">In the context of SQLAlchemy, the term “released”
refers to the process of ending the usage of a particular
database connection. SQLAlchemy features the usage
of connection pools, which allows configurability as to
the lifespan of database connections. When using a pooled
connection, the process of “closing” it, i.e. invoking
a statement like <tt class="docutils literal"><span class="pre">connection.close()</span></tt>, may have the effect
of the connection being returned to an existing pool,
or it may have the effect of actually shutting down the
underlying TCP/IP connection referred to by that connection -
which one takes place depends on configuration as well
as the current state of the pool. So we used the term
<em>released</em> instead, to mean “do whatever it is you do
with connections when we’re done using them”.</p>
<p>The term will sometimes be used in the phrase, “release
transactional resources”, to indicate more explicitly that
what we are actually “releasing” is any transactional
state which as accumulated upon the connection. In most
situations, the proces of selecting from tables, emitting
updates, etc. acquires <a class="reference internal" href="#term-isolated"><em class="xref std std-term">isolated</em></a> state upon
that connection as well as potential row or table locks.
This state is all local to a particular transaction
on the connection, and is released when we emit a rollback.
An important feature of the connection pool is that when
we return a connection to the pool, the <tt class="docutils literal"><span class="pre">connection.rollback()</span></tt>
method of the DBAPI is called as well, so that as the
connection is set up to be used again, it’s in a “clean”
state with no references held to the previous series
of operations.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="core/pooling.html"><em>Connection Pooling</em></a></p>
</div>
</dd>
<dt id="term-returning">RETURNING</dt>
<dd><p class="first">This is a non-SQL standard clause provided in various forms by
certain backends, which provides the service of returning a result
set upon execution of an INSERT, UPDATE or DELETE statement. Any set
of columns from the matched rows can be returned, as though they were
produced from a SELECT statement.</p>
<p>The RETURNING clause provides both a dramatic performance boost to
common update/select scenarios, including retrieval of inline- or
default- generated primary key values and defaults at the moment they
were created, as well as a way to get at server-generated
default values in an atomic way.</p>
<p>An example of RETURNING, idiomatic to Postgresql, looks like:</p>
<div class="highlight-python"><pre>INSERT INTO user_account (name) VALUES ('new name') RETURNING id, timestamp</pre>
</div>
<p>Above, the INSERT statement will provide upon execution a result set
which includes the values of the columns <tt class="docutils literal"><span class="pre">user_account.id</span></tt> and
<tt class="docutils literal"><span class="pre">user_account.timestamp</span></tt>, which above should have been generated as default
values as they are not included otherwise (but note any series of columns
or SQL expressions can be placed into RETURNING, not just default-value columns).</p>
<p>The backends that currently support
RETURNING or a similar construct are Postgresql, SQL Server, Oracle,
and Firebird. The Postgresql and Firebird implementations are generally
full featured, whereas the implementations of SQL Server and Oracle
have caveats. On SQL Server, the clause is known as “OUTPUT INSERTED”
for INSERT and UPDATE statements and “OUTPUT DELETED” for DELETE statements;
the key caveat is that triggers are not supported in conjunction with this
keyword. On Oracle, it is known as “RETURNING...INTO”, and requires that the
value be placed into an OUT paramter, meaning not only is the syntax awkward,
but it can also only be used for one row at a time.</p>
<p class="last">SQLAlchemy’s <a class="reference internal" href="core/dml.html#sqlalchemy.sql.expression.UpdateBase.returning" title="sqlalchemy.sql.expression.UpdateBase.returning"><tt class="xref py py-meth docutils literal"><span class="pre">UpdateBase.returning()</span></tt></a> system provides a layer of abstraction
on top of the RETURNING systems of these backends to provide a consistent
interface for returning columns. The ORM also includes many optimizations
that make use of RETURNING when available.</p>
</dd>
<dt id="term-session">Session</dt>
<dd><p class="first">The container or scope for ORM database operations. Sessions
load instances from the database, track changes to mapped
instances and persist changes in a single unit of work when
flushed.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/session.html"><em>Using the Session</em></a></p>
</div>
</dd>
<dt id="term-subquery">subquery</dt>
<dd><p class="first">Refers to a <tt class="docutils literal"><span class="pre">SELECT</span></tt> statement that is embedded within an enclosing
<tt class="docutils literal"><span class="pre">SELECT</span></tt>.</p>
<p>A subquery comes in two general flavors, one known as a “scalar select”
which specifically must return exactly one row and one column, and the
other form which acts as a “derived table” and serves as a source of
rows for the FROM clause of another select. A scalar select is eligible
to be placed in the <a class="reference internal" href="#term-where-clause"><em class="xref std std-term">WHERE clause</em></a>, <a class="reference internal" href="#term-columns-clause"><em class="xref std std-term">columns clause</em></a>,
ORDER BY clause or HAVING clause of the enclosing select, whereas the
derived table form is eligible to be placed in the FROM clause of the
enclosing <tt class="docutils literal"><span class="pre">SELECT</span></tt>.</p>
<p>Examples:</p>
<ol class="last arabic">
<li><p class="first">a scalar subquery placed in the <a class="reference internal" href="#term-columns-clause"><em class="xref std std-term">columns clause</em></a> of an enclosing
<tt class="docutils literal"><span class="pre">SELECT</span></tt>. The subquery in this example is a <a class="reference internal" href="#term-correlated-subquery"><em class="xref std std-term">correlated subquery</em></a> because part
of the rows which it selects from are given via the enclosing statement.</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">id</span><span class="p">,</span> <span class="p">(</span><span class="k">SELECT</span> <span class="n">name</span> <span class="k">FROM</span> <span class="n">address</span> <span class="k">WHERE</span> <span class="n">address</span><span class="p">.</span><span class="n">user_id</span><span class="o">=</span><span class="k">user</span><span class="p">.</span><span class="n">id</span><span class="p">)</span>
<span class="k">FROM</span> <span class="k">user</span></pre></div>
</div>
</li>
<li><p class="first">a scalar subquery placed in the <a class="reference internal" href="#term-where-clause"><em class="xref std std-term">WHERE clause</em></a> of an enclosing
<tt class="docutils literal"><span class="pre">SELECT</span></tt>. This subquery in this example is not correlated as it selects a fixed result.</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">id</span><span class="p">,</span> <span class="n">name</span> <span class="k">FROM</span> <span class="k">user</span>
<span class="k">WHERE</span> <span class="n">status</span><span class="o">=</span><span class="p">(</span><span class="k">SELECT</span> <span class="n">status_id</span> <span class="k">FROM</span> <span class="n">status_code</span> <span class="k">WHERE</span> <span class="n">code</span><span class="o">=</span><span class="s1">'C'</span><span class="p">)</span></pre></div>
</div>
</li>
<li><p class="first">a derived table subquery placed in the <a class="reference internal" href="#term-from-clause"><em class="xref std std-term">FROM clause</em></a> of an enclosing
<tt class="docutils literal"><span class="pre">SELECT</span></tt>. Such a subquery is almost always given an alias name.</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="k">user</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="k">user</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">ad_subq</span><span class="p">.</span><span class="n">email_address</span>
<span class="k">FROM</span>
<span class="k">user</span> <span class="k">JOIN</span>
<span class="p">(</span><span class="k">select</span> <span class="n">user_id</span><span class="p">,</span> <span class="n">email_address</span> <span class="k">FROM</span> <span class="n">address</span> <span class="k">WHERE</span> <span class="n">address_type</span><span class="o">=</span><span class="s1">'Q'</span><span class="p">)</span> <span class="k">AS</span> <span class="n">ad_subq</span>
<span class="k">ON</span> <span class="k">user</span><span class="p">.</span><span class="n">id</span> <span class="o">=</span> <span class="n">ad_subq</span><span class="p">.</span><span class="n">user_id</span></pre></div>
</div>
</li>
</ol>
</dd>
<dt id="term-transient">transient</dt>
<dd><p class="first">This describes one of the four major object states which
an object can have within a <a class="reference internal" href="#term-session"><em class="xref std std-term">session</em></a>; a transient object
is a new object that doesn’t have any database identity
and has not been associated with a session yet. When the
object is added to the session, it moves to the
<a class="reference internal" href="#term-pending"><em class="xref std std-term">pending</em></a> state.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="orm/session.html#session-object-states"><em>Quickie Intro to Object States</em></a></p>
</div>
</dd>
<dt id="term-unique-constraint"><span id="term-unique-key-index"></span>unique constraint<br />unique key index</dt>
<dd><p class="first">A unique key index can uniquely identify each row of data
values in a database table. A unique key index comprises a
single column or a set of columns in a single database table.
No two distinct rows or data records in a database table can
have the same data value (or combination of data values) in
those unique key index columns if NULL values are not used.
Depending on its design, a database table may have many unique
key indexes but at most one primary key index.</p>
<p>(via Wikipedia)</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://en.wikipedia.org/wiki/Unique_key#Defining_unique_keys">http://en.wikipedia.org/wiki/Unique_key#Defining_unique_keys</a></p>
</div>
</dd>
<dt id="term-unit-of-work">unit of work</dt>
<dd><p class="first">This pattern is where the system transparently keeps
track of changes to objects and periodically flushes all those
pending changes out to the database. SQLAlchemy’s Session
implements this pattern fully in a manner similar to that of
Hibernate.</p>
<div class="last admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference external" href="http://martinfowler.com/eaaCatalog/unitOfWork.html">Unit of Work by Martin Fowler</a></p>
<p class="last"><a class="reference internal" href="orm/session.html"><em>Using the Session</em></a></p>
</div>
</dd>
<dt id="term-where-clause">WHERE clause</dt>
<dd><p class="first">The portion of the <tt class="docutils literal"><span class="pre">SELECT</span></tt> statement which indicates criteria
by which rows should be filtered. It is a single SQL expression
which follows the keyword <tt class="docutils literal"><span class="pre">WHERE</span></tt>.</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">user_account</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">user_account</span><span class="p">.</span><span class="n">email</span>
<span class="k">FROM</span> <span class="n">user_account</span>
<span class="k">WHERE</span> <span class="n">user_account</span><span class="p">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'fred'</span> <span class="k">AND</span> <span class="n">user_account</span><span class="p">.</span><span class="n">status</span> <span class="o">=</span> <span class="s1">'E'</span></pre></div>
</div>
<p class="last">Above, the phrase <tt class="docutils literal"><span class="pre">WHERE</span> <span class="pre">user_account.name</span> <span class="pre">=</span> <span class="pre">'fred'</span> <span class="pre">AND</span> <span class="pre">user_account.status</span> <span class="pre">=</span> <span class="pre">'E'</span></tt>
comprises the WHERE clause of the <tt class="docutils literal"><span class="pre">SELECT</span></tt>.</p>
</dd>
</dl>
</div>
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