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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="xfunc-sql"></a>32.4.Query Language (<acronym class="acronym">SQL</acronym>) Functions</h2></div></div></div>
<a name="id704141"></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 &#8220;<span class="quote">the first row</span>&#8221; of a multirow
    result is not well-defined unless you use <code class="literal">ORDER BY</code>.)
    If the last query happens
    to return no rows at all, the null value will be returned.
   </p>
<p>    <a name="id704179"></a> Alternatively,
    an SQL function may be declared to return a set, by specifying the
    function's return type as <code class="literal">SETOF
    <em class="replaceable"><code>sometype</code></em></code>.  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 must be a list of SQL
    statements separated by semicolons.  A semicolon after the last
    statement is optional.  Unless the function is declared to return
    <code class="type">void</code>, the last statement must be a <code class="command">SELECT</code>.
   </p>
<p>     Any collection of commands in the  <acronym class="acronym">SQL</acronym>
     language can be packaged together and defined as a function.
     Besides <code class="command">SELECT</code> queries, the commands can include data
     modification queries (<code class="command">INSERT</code>,
     <code class="command">UPDATE</code>, and <code class="command">DELETE</code>), as well as
     other SQL commands. (The only exception is that you can't put
     <code class="command">BEGIN</code>, <code class="command">COMMIT</code>, <code class="command">ROLLBACK</code>, or
     <code class="command">SAVEPOINT</code> commands into a <acronym class="acronym">SQL</acronym> function.)
     However, the final command 
     must be a <code class="command">SELECT</code> 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 <code class="type">void</code>.
     In that case, the function body must not end with a <code class="command">SELECT</code>.
     For example, this function removes rows with negative salaries from
     the <code class="literal">emp</code> table:

</p>
<pre class="screen">CREATE FUNCTION clean_emp() RETURNS void AS '
    DELETE FROM emp
        WHERE salary &lt; 0;
' LANGUAGE SQL;

SELECT clean_emp();

 clean_emp
-----------

(1 row)</pre>
<p>
    </p>
<p>    The syntax of the <code class="command">CREATE FUNCTION</code> command requires
    the function body to be written as a string constant.  It is usually
    most convenient to use dollar quoting (see <a href="sql-syntax.html#sql-syntax-dollar-quoting" title="4.1.2.2.Dollar-Quoted String Constants">Section4.1.2.2, &#8220;Dollar-Quoted String Constants&#8221;</a>) for the string constant.
    If you choose to use regular single-quoted string constant syntax,
    you must escape single quote marks (<code class="literal">'</code>) and backslashes
    (<code class="literal">\</code>) used in the body of the function, typically by
    doubling them (see <a href="sql-syntax.html#sql-syntax-strings" title="4.1.2.1.String Constants">Section4.1.2.1, &#8220;String Constants&#8221;</a>).
   </p>
<p>    Arguments to the SQL function are referenced in the function
    body using the syntax <code class="literal">$<em class="replaceable"><code>n</code></em></code>: <code class="literal">$1</code>
    refers to the first argument, <code class="literal">$2</code> to the second, and so on.
    If an argument is of a composite type, then the dot notation,
    e.g., <code class="literal">$1.name</code>, may be used to access attributes
    of the argument.  The arguments can only be used as data values,
    not as identifiers.  Thus for example this is reasonable:
</p>
<pre class="programlisting">INSERT INTO mytable VALUES ($1);</pre>
<p>
but this will not work:
</p>
<pre class="programlisting">INSERT INTO $1 VALUES (42);</pre>
<p>
   </p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-sql-base-functions"></a>32.4.1.<acronym class="acronym">SQL</acronym> Functions on Base Types</h3></div></div></div>
<p>     The simplest possible <acronym class="acronym">SQL</acronym> function has no arguments and
     simply returns a base type, such as <code class="type">integer</code>:
     
</p>
<pre class="screen">CREATE FUNCTION one() RETURNS integer AS $$
    SELECT 1 AS result;
$$ LANGUAGE SQL;

-- Alternative syntax for string literal:
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 <code class="literal">result</code>),  but this column alias is not visible
     outside the function.  Hence,  the  result  is labeled <code class="literal">one</code>
     instead of <code class="literal">result</code>.
    </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 <code class="literal">$1</code>
     and <code class="literal">$2</code>.

</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>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id704549"></a>32.4.2.<acronym class="acronym">SQL</acronym> Functions on Composite Types</h3></div></div></div>
<p>     When writing  functions with arguments of composite
     types, we must  not  only  specify  which
     argument  we  want (as we did above with <code class="literal">$1</code> and <code class="literal">$2</code>) but
     also the desired attribute (field) of  that  argument.   For  example,
     suppose that 
     <code class="type">emp</code> 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      numeric,
    age         integer,
    cubicle     point
);

CREATE FUNCTION double_salary(emp) RETURNS numeric 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
------+-------
 Bill |  8400</pre>
<p>
    </p>
<p>     Notice the use of the syntax <code class="literal">$1.salary</code>
     to select one field of the argument row value.  Also notice
     how the calling <code class="command">SELECT</code> command uses <code class="literal">*</code>
     to select
     the entire current row of a table as a composite value.  The table
     row can alternatively be referenced using just the table name,
     like this:
</p>
<pre class="screen">SELECT name, double_salary(emp) AS dream
    FROM emp
    WHERE emp.cubicle ~= point '(2,1)';</pre>
<p>
     but this usage is deprecated since it's easy to get confused.
    </p>
<p>     Sometimes it is handy to construct a composite argument value
     on-the-fly.  This can be done with the <code class="literal">ROW</code> construct.
     For example, we could adjust the data being passed to the function:
</p>
<pre class="screen">SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream
    FROM emp;</pre>
<p>
    </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 <code class="type">emp</code> row:

</p>
<pre class="programlisting">CREATE FUNCTION new_emp() RETURNS emp AS $$
    SELECT text 'None' AS name,
        1000.0 AS salary,
        25 AS age,
        point '(2,2)' AS cubicle;
$$ 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>
<div class="itemizedlist"><ul type="disc">
<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"><code class="computeroutput">ERROR:  function declared to return emp returns varchar instead of text at column 1</code></pre>
<p>
       </p>
</li>
</ul></div>
<p>
    </p>
<p>     A different way to define the same function is:

</p>
<pre class="programlisting">CREATE FUNCTION new_emp() RETURNS emp AS $$
    SELECT ROW('None', 1000.0, 25, '(2,2)')::emp;
$$ LANGUAGE SQL;</pre>
<p>

     Here we wrote a <code class="command">SELECT</code> that returns just a single
     column of the correct composite type.  This isn't really better
     in this situation, but it is a handy alternative in some cases
     [mdash ] for example, if we need to compute the result by calling
     another function that returns the desired composite value.
    </p>
<p>     We could call this function directly in either of two ways:

</p>
<pre class="screen">SELECT new_emp();

         new_emp
--------------------------
 (None,1000.0,25,"(2,2)")

SELECT * FROM new_emp();

 name | salary | age | cubicle
------+--------+-----+---------
 None | 1000.0 |  25 | (2,2)</pre>
<p>

     The second way is described more fully in <a href="xfunc-sql.html#xfunc-sql-table-functions" title="32.4.4.SQL Functions as Table Sources">Section32.4.4, &#8220;<acronym class="acronym">SQL</acronym> Functions as Table Sources&#8221;</a>.
    </p>
<p>     When you use a function that returns a composite type,
     you might want only one field (attribute) from its result.
     You can do that with syntax like this:

</p>
<pre class="screen">SELECT (new_emp()).name;

 name
------
 None</pre>
<p>

     The extra parentheses are needed to keep the parser from getting
     confused.  If you try to do it without them, you get something like this:

</p>
<pre class="screen">SELECT new_emp().name;
ERROR:  syntax error at or near "." at character 17
LINE 1: SELECT new_emp().name;
                        ^</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 <code class="literal">attribute(table)</code>  and  <code class="literal">table.attribute</code>
     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 &lt; 30;

SELECT name(emp) AS youngster FROM emp WHERE age(emp) &lt; 30;

 youngster
-----------
 Sam
 Andy</pre>
<p>
    </p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>      The equivalence between functional notation and attribute notation
      makes it possible to use functions on composite types to emulate
      &#8220;<span class="quote">computed fields</span>&#8221;.
      <a name="id704830"></a>
      <a name="id704838"></a>
      For example, using the previous definition
      for <code class="literal">double_salary(emp)</code>, we can write

</p>
<pre class="screen">SELECT emp.name, emp.double_salary FROM emp;</pre>
<p>

      An application using this wouldn't need to be directly aware that
      <code class="literal">double_salary</code> isn't a real column of the table.
      (You can also emulate computed fields with views.)
     </p>
</div>
<p>     Another way to use a function returning a composite type is to pass the
     result to another function that accepts the correct row type as input:

</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>
<p>     Still another way to use a function that returns a composite type is to
     call it as a table function, as described in <a href="xfunc-sql.html#xfunc-sql-table-functions" title="32.4.4.SQL Functions as Table Sources">Section32.4.4, &#8220;<acronym class="acronym">SQL</acronym> Functions as Table Sources&#8221;</a>.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-output-parameters"></a>32.4.3.Functions with Output Parameters</h3></div></div></div>
<a name="id704909"></a><p>     An alternative way of describing a function's results is to define it
     with <em class="firstterm">output parameters</em>, as in this example:

</p>
<pre class="screen">CREATE FUNCTION add_em (IN x int, IN y int, OUT sum int)
AS 'SELECT $1 + $2'
LANGUAGE SQL;

SELECT add_em(3,7);
 add_em
--------
     10
(1 row)</pre>
<p>

     This is not essentially different from the version of <code class="literal">add_em</code>
     shown in <a href="xfunc-sql.html#xfunc-sql-base-functions" title="32.4.1.SQL Functions on Base Types">Section32.4.1, &#8220;<acronym class="acronym">SQL</acronym> Functions on Base Types&#8221;</a>.  The real value of
     output parameters is that they provide a convenient way of defining
     functions that return several columns.  For example,

</p>
<pre class="screen">CREATE FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int)
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;

 SELECT * FROM sum_n_product(11,42);
 sum | product
-----+---------
  53 |     462
(1 row)</pre>
<p>

     What has essentially happened here is that we have created an anonymous
     composite type for the result of the function.  The above example has
     the same end result as

</p>
<pre class="screen">CREATE TYPE sum_prod AS (sum int, product int);

CREATE FUNCTION sum_n_product (int, int) RETURNS sum_prod
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;</pre>
<p>

     but not having to bother with the separate composite type definition
     is often handy.
    </p>
<p>     Notice that output parameters are not included in the calling argument
     list when invoking such a function from SQL.  This is because
     <span class="productname">PostgreSQL</span> considers only the input
     parameters to define the function's calling signature.  That means
     also that only the input parameters matter when referencing the function
     for purposes such as dropping it.  We could drop the above function
     with either of

</p>
<pre class="screen">DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int);
DROP FUNCTION sum_n_product (int, int);</pre>
<p>
    </p>
<p>     Parameters can be marked as <code class="literal">IN</code> (the default),
     <code class="literal">OUT</code>, or <code class="literal">INOUT</code>.  An <code class="literal">INOUT</code>
     parameter serves as both an input parameter (part of the calling
     argument list) and an output parameter (part of the result record type).
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-sql-table-functions"></a>32.4.4.<acronym class="acronym">SQL</acronym> Functions as Table Sources</h3></div></div></div>
<p>     All SQL functions may be used in the <code class="literal">FROM</code> 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
(1 row)</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 <code class="literal">SETOF</code>.  That is described in the next section.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id705091"></a>32.4.5.<acronym class="acronym">SQL</acronym> Functions Returning Sets</h3></div></div></div>
<p>     When an SQL function is declared as returning <code class="literal">SETOF
     <em class="replaceable"><code>sometype</code></em></code>, the function's final
     <code class="command">SELECT</code> 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 <code class="literal">FROM</code>
     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 <code class="literal">foo</code> 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 <code class="command">SELECT</code>,
     notice that no output row appears for <code class="literal">Child2</code>, <code class="literal">Child3</code>, 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" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id705221"></a>32.4.6.Polymorphic <acronym class="acronym">SQL</acronym> Functions</h3></div></div></div>
<p>     <acronym class="acronym">SQL</acronym> functions may be declared to accept and
     return the polymorphic types <code class="type">anyelement</code> and
     <code class="type">anyarray</code>.  See <a href="extend-type-system.html#extend-types-polymorphic" title="32.2.5.Polymorphic Types">Section32.2.5, &#8220;Polymorphic Types&#8221;</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 <code class="literal">'a'::text</code>
     to specify that the argument is of type <code class="type">text</code>. This is
     required if the argument is just a string literal, since otherwise
     it would be treated as type
     <code class="type">unknown</code>, and array of <code class="type">unknown</code> is not a valid
     type.
     Without the typecast, you will get errors like this:
</p>
<pre class="screen"><code class="computeroutput">ERROR:  could not determine "anyarray"/"anyelement" type because input has type "unknown"</code></pre>
<p>
    </p>
<p>     It is permitted to have polymorphic arguments with a fixed
     return type, but the converse is not. For example:
</p>
<pre class="screen">CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
    SELECT $1 &gt; $2;
$$ 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>
    </p>
<p>     Polymorphism can be used with functions that have output arguments.
     For example:
</p>
<pre class="screen">CREATE FUNCTION dup (f1 anyelement, OUT f2 anyelement, OUT f3 anyarray)
AS 'select $1, array[$1,$1]' LANGUAGE sql;

SELECT * FROM dup(22);
 f2 |   f3
----+---------
 22 | {22,22}
(1 row)</pre>
<p>
    </p>
</div>
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