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>Chapter 33. Extending <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
></TD
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><H1
CLASS="SECT1"
><A
NAME="XFUNC-SQL"
>33.4. Query Language (<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>) Functions</A
></H1
><A
NAME="AEN28738"
></A
><P
> SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list.
In the simple (non-set)
case, the first row of the last query's result will be returned.
(Bear in mind that <SPAN
CLASS="QUOTE"
>"the first row"</SPAN
> of a multirow
result is not well-defined unless you use <TT
CLASS="LITERAL"
>ORDER BY</TT
>.)
If the last query happens
to return no rows at all, the null value will be returned.
</P
><P
> <A
NAME="AEN28746"
></A
> Alternatively,
an SQL function may be declared to return a set, by specifying the
function's return type as <TT
CLASS="LITERAL"
>SETOF
<VAR
CLASS="REPLACEABLE"
>sometype</VAR
></TT
>.<A
NAME="AEN28751"
></A
>
In this case all rows of the last query's result are returned.
Further details appear below.
</P
><P
> The body of an SQL function should be a list of one or more SQL
statements separated by semicolons. Note that because the syntax
of the <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> command requires the body of the
function to be enclosed in single quotes, single quote marks
(<TT
CLASS="LITERAL"
>'</TT
>) used
in the body of the function must be escaped, by writing two single
quotes (<TT
CLASS="LITERAL"
>''</TT
>) or a backslash (<TT
CLASS="LITERAL"
>\'</TT
>) where each
quote is desired.
</P
><P
> Arguments to the SQL function may be referenced in the function
body using the syntax <TT
CLASS="LITERAL"
>$<VAR
CLASS="REPLACEABLE"
>n</VAR
></TT
>: <TT
CLASS="LITERAL"
>$1</TT
> refers to
the first argument, <TT
CLASS="LITERAL"
>$2</TT
> to the second, and so on. If an argument
is of a composite type, then the dot notation,
e.g., <TT
CLASS="LITERAL"
>$1.name</TT
>, may be used to access attributes
of the argument.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN28764"
>33.4.1. <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Functions on Base Types</A
></H2
><P
> The simplest possible <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> function has no arguments and
simply returns a base type, such as <TT
CLASS="TYPE"
>integer</TT
>:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION one() RETURNS integer AS '
SELECT 1 AS result;
' LANGUAGE SQL;
SELECT one();
one
-----
1</PRE
><P>
</P
><P
> Notice that we defined a column alias within the function body for the result of the function
(with the name <TT
CLASS="LITERAL"
>result</TT
>), but this column alias is not visible
outside the function. Hence, the result is labeled <TT
CLASS="LITERAL"
>one</TT
>
instead of <TT
CLASS="LITERAL"
>result</TT
>.
</P
><P
> It is almost as easy to define <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> functions
that take base types as arguments. In the example below, notice
how we refer to the arguments within the function as <TT
CLASS="LITERAL"
>$1</TT
>
and <TT
CLASS="LITERAL"
>$2</TT
>.
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION add_em(integer, integer) RETURNS integer AS '
SELECT $1 + $2;
' LANGUAGE SQL;
SELECT add_em(1, 2) AS answer;
answer
--------
3</PRE
><P>
</P
><P
> Here is a more useful function, which might be used to debit a
bank account:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS '
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT 1;
' LANGUAGE SQL;</PRE
><P>
A user could execute this function to debit account 17 by $100.00 as
follows:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT tf1(17, 100.0);</PRE
><P>
</P
><P
> In practice one would probably like a more useful result from the
function than a constant 1, so a more likely definition
is
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS '
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT balance FROM bank WHERE accountno = $1;
' LANGUAGE SQL;</PRE
><P>
which adjusts the balance and returns the new balance.
</P
><P
> Any collection of commands in the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
language can be packaged together and defined as a function.
Besides <TT
CLASS="COMMAND"
>SELECT</TT
> queries,
the commands can include data modification (i.e.,
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, and
<TT
CLASS="COMMAND"
>DELETE</TT
>). However, the final command
must be a <TT
CLASS="COMMAND"
>SELECT</TT
> that returns whatever is
specified as the function's return type. Alternatively, if you
want to define a SQL function that performs actions but has no
useful value to return, you can define it as returning <TT
CLASS="TYPE"
>void</TT
>.
In that case, the function body must not end with a <TT
CLASS="COMMAND"
>SELECT</TT
>.
For example:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION clean_emp() RETURNS void AS '
DELETE FROM emp
WHERE salary <= 0;
' LANGUAGE SQL;
SELECT clean_emp();
clean_emp
-----------
(1 row)</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN28795"
>33.4.2. <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Functions on Composite Types</A
></H2
><P
> When specifying functions with arguments of composite
types, we must not only specify which
argument we want (as we did above with <TT
CLASS="LITERAL"
>$1</TT
> and <TT
CLASS="LITERAL"
>$2</TT
>) but
also the attributes of that argument. For example, suppose that
<TT
CLASS="TYPE"
>emp</TT
> is a table containing employee data, and therefore
also the name of the composite type of each row of the table. Here
is a function <CODE
CLASS="FUNCTION"
>double_salary</CODE
> that computes what someone's
salary would be if it were doubled:
</P><PRE
CLASS="SCREEN"
>CREATE TABLE emp (
name text,
salary integer,
age integer,
cubicle point
);
CREATE FUNCTION double_salary(emp) RETURNS integer AS '
SELECT $1.salary * 2 AS salary;
' LANGUAGE SQL;
SELECT name, double_salary(emp) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
name | dream
------+-------
Sam | 2400</PRE
><P>
</P
><P
> Notice the use of the syntax <TT
CLASS="LITERAL"
>$1.salary</TT
>
to select one field of the argument row value. Also notice
how the calling <TT
CLASS="COMMAND"
>SELECT</TT
> command uses a table name to denote
the entire current row of that table as a composite value. The table
row can alternatively be referenced like this:
</P><PRE
CLASS="SCREEN"
>SELECT name, double_salary(emp.*) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';</PRE
><P>
which emphasizes its row nature.
</P
><P
> It is also possible to build a function that returns a composite type.
This is an example of a function
that returns a single <TT
CLASS="TYPE"
>emp</TT
> row:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION new_emp() RETURNS emp AS '
SELECT text ''None'' AS name,
1000 AS salary,
25 AS age,
point ''(2,2)'' AS cubicle;
' LANGUAGE SQL;</PRE
><P>
In this example we have specified each of the attributes
with a constant value, but any computation
could have been substituted for these constants.
</P
><P
> Note two important things about defining the function:
<P
></P
></P><UL
><LI
><P
> The select list order in the query must be exactly the same as
that in which the columns appear in the table associated
with the composite type. (Naming the columns, as we did above,
is irrelevant to the system.)
</P
></LI
><LI
><P
> You must typecast the expressions to match the
definition of the composite type, or you will get errors like this:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="COMPUTEROUTPUT"
>ERROR: function declared to return emp returns varchar instead of text at column 1</SAMP
></PRE
><P>
</P
></LI
></UL
><P>
</P
><P
> A function that returns a row (composite type) can be used as a table
function, as described below. It can also be called in the context
of an SQL expression, but only when you
extract a single attribute out of the row or pass the entire row into
another function that accepts the same composite type.
</P
><P
> This is an example of extracting an attribute out of a row type:
</P><PRE
CLASS="SCREEN"
>SELECT (new_emp()).name;
name
------
None</PRE
><P>
We need the extra parentheses to keep the parser from getting confused:
</P><PRE
CLASS="SCREEN"
>SELECT new_emp().name;
ERROR: syntax error at or near "." at character 17</PRE
><P>
</P
><P
> Another option is to use
functional notation for extracting an attribute. The simple way
to explain this is that we can use the
notations <TT
CLASS="LITERAL"
>attribute(table)</TT
> and <TT
CLASS="LITERAL"
>table.attribute</TT
>
interchangeably.
</P><PRE
CLASS="SCREEN"
>SELECT name(new_emp());
name
------
None</PRE
><P>
</P><PRE
CLASS="SCREEN"
>-- This is the same as:
-- SELECT emp.name AS youngster FROM emp WHERE emp.age < 30
SELECT name(emp) AS youngster
FROM emp
WHERE age(emp) < 30;
youngster
-----------
Sam</PRE
><P>
</P
><P
> The other way to use a function returning a row result is to declare a
second function accepting a row type argument and pass the
result of the first function to it:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION getname(emp) RETURNS text AS '
SELECT $1.name;
' LANGUAGE SQL;
SELECT getname(new_emp());
getname
---------
None
(1 row)</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN28830"
>33.4.3. <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Functions as Table Sources</A
></H2
><P
> All SQL functions may be used in the <TT
CLASS="LITERAL"
>FROM</TT
> clause of a query,
but it is particularly useful for functions returning composite types.
If the function is defined to return a base type, the table function
produces a one-column table. If the function is defined to return
a composite type, the table function produces a column for each attribute
of the composite type.
</P
><P
> Here is an example:
</P><PRE
CLASS="SCREEN"
>CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');
CREATE FUNCTION getfoo(int) RETURNS foo AS '
SELECT * FROM foo WHERE fooid = $1;
' LANGUAGE SQL;
SELECT *, upper(fooname) FROM getfoo(1) AS t1;
fooid | foosubid | fooname | upper
-------+----------+---------+-------
1 | 1 | Joe | JOE
(2 rows)</PRE
><P>
As the example shows, we can work with the columns of the function's
result just the same as if they were columns of a regular table.
</P
><P
> Note that we only got one row out of the function. This is because
we did not use <TT
CLASS="LITERAL"
>SETOF</TT
>. This is described in the next section.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN28839"
>33.4.4. <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Functions Returning Sets</A
></H2
><P
> When an SQL function is declared as returning <TT
CLASS="LITERAL"
>SETOF
<VAR
CLASS="REPLACEABLE"
>sometype</VAR
></TT
>, the function's final
<TT
CLASS="COMMAND"
>SELECT</TT
> query is executed to completion, and each row it
outputs is returned as an element of the result set.
</P
><P
> This feature is normally used when calling the function in the <TT
CLASS="LITERAL"
>FROM</TT
>
clause. In this case each row returned by the function becomes
a row of the table seen by the query. For example, assume that
table <TT
CLASS="LITERAL"
>foo</TT
> has the same contents as above, and we say:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS '
SELECT * FROM foo WHERE fooid = $1;
' LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;</PRE
><P>
Then we would get:
</P><PRE
CLASS="SCREEN"
> fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)</PRE
><P>
</P
><P
> Currently, functions returning sets may also be called in the select list
of a query. For each row that the query
generates by itself, the function returning set is invoked, and an output
row is generated for each element of the function's result set. Note,
however, that this capability is deprecated and may be removed in future
releases. The following is an example function returning a set from the
select list:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION listchildren(text) RETURNS SETOF text AS
'SELECT name FROM nodes WHERE parent = $1'
LANGUAGE SQL;
SELECT * FROM nodes;
name | parent
-----------+--------
Top |
Child1 | Top
Child2 | Top
Child3 | Top
SubChild1 | Child1
SubChild2 | Child1
(6 rows)
SELECT listchildren('Top');
listchildren
--------------
Child1
Child2
Child3
(3 rows)
SELECT name, listchildren(name) FROM nodes;
name | listchildren
--------+--------------
Top | Child1
Top | Child2
Top | Child3
Child1 | SubChild1
Child1 | SubChild2
(5 rows)</PRE
><P>
In the last <TT
CLASS="COMMAND"
>SELECT</TT
>,
notice that no output row appears for <TT
CLASS="LITERAL"
>Child2</TT
>, <TT
CLASS="LITERAL"
>Child3</TT
>, etc.
This happens because <CODE
CLASS="FUNCTION"
>listchildren</CODE
> returns an empty set
for those arguments, so no result rows are generated.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN28857"
>33.4.5. Polymorphic <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Functions</A
></H2
><P
> <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> functions may be declared to accept and
return the polymorphic types <TT
CLASS="TYPE"
>anyelement</TT
> and
<TT
CLASS="TYPE"
>anyarray</TT
>. See <A
HREF="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC"
>Section 33.2.5</A
> for a more detailed
explanation of polymorphic functions. Here is a polymorphic
function <CODE
CLASS="FUNCTION"
>make_array</CODE
> that builds up an array
from two arbitrary data type elements:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS '
SELECT ARRAY[$1, $2];
' LANGUAGE SQL;
SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
intarray | textarray
----------+-----------
{1,2} | {a,b}
(1 row)</PRE
><P>
</P
><P
> Notice the use of the typecast <TT
CLASS="LITERAL"
>'a'::text</TT
>
to specify that the argument is of type <TT
CLASS="TYPE"
>text</TT
>. This is
required if the argument is just a string literal, since otherwise
it would be treated as type
<TT
CLASS="TYPE"
>unknown</TT
>, and array of <TT
CLASS="TYPE"
>unknown</TT
> is not a valid
type.
Without the typecast, you will get errors like this:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="COMPUTEROUTPUT"
>ERROR: could not determine "anyarray"/"anyelement" type because input has type "unknown"</SAMP
></PRE
><P>
</P
><P
> It is permitted to have polymorphic arguments with a deterministic
return type, but the converse is not. For example:
</P><PRE
CLASS="SCREEN"
>CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS '
SELECT $1 > $2;
' LANGUAGE SQL;
SELECT is_greater(1, 2);
is_greater
------------
f
(1 row)
CREATE FUNCTION invalid_func() RETURNS anyelement AS '
SELECT 1;
' LANGUAGE SQL;
ERROR: cannot determine result data type
DETAIL: A function returning "anyarray" or "anyelement" must have at least one argument of either type.</PRE
><P>
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