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<title>Problems Of String Based Query Languages</title>
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<h1>Problems Of String Based Query Languages</h1>
<p>Let's look how a query can be expressed against the Car
class in some of the object querying languages:<br></p>
<p><b>OQL</b> </p>
<p><code>String oql = "select *
from pilot in AllPilots where pilot.points < 100";</code></p>
<p><code>OQLQuery query = new
OQLQuery(oql); </code></p>
<p><code>Object pilots =
query.execute();</code></p>
<p><b>JDOQL</b> </p>
<p><code>Query query =
persistenceManager.newQuery(Pilot.class, "points < 100");</code></p>
<p><code>Collection pilots =
(Collection)query.execute();</code></p>
<p><b>db4o SODA, using C#</b> </p>
<p><code>Query query =
database.Query();</code></p>
<p><code>query.Constrain(typeof(Pilot));
</code></p>
<p><code>query.Descend("points").Constrain(100).Smaller();</code></p>
<p><code>IList pilots =
query.Execute();</code></p>
<p>As you can see, query parameters ("points") and constraints
("<100") are expressed as strings, which results in the following problems:</p>
<ul><li>Modern
development environments do not check embedded strings for syntactic and
semantic correctness. A small typo in a query (for example 10 instead of
100) will be very difficult trace, moreover it can pass unnoticed to a
production version.</li><li>Embedded
strings are not affected by refactoring tools. As the application evolves
and the changes are made to the field variables, the string-based queries
will become obsolete and will need to be changed by hand.</li><li>String
queries address fields directly instead of using publicly accessible
methods/attributes, breaking encapsulation principle.</li><li>Embedded
strings can be on the way of modern agile techniques, which encourage
constant refactoring. Since string queries are difficult to refactor and
maintain they will delay a decision to refactor and result in a
lower-quality code.</li><li>Working
with a string query language inside an OO language requires a developer to
learn both and switch between them in the development cycle. </li><li>Reusability
support of OO languages (method calls, polymorphism, overriding) are not
accessible to string-based queries.</li><li>Embedded
strings can be subject to injection attacks.</li></ul></div>
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This revision (1) was last Modified 2007-06-02T18:25:48 by Tetyana.
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