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 “the first row” of a multirow
result is not well-defined unless you use ORDER BY
.)
If the last query happens
to return no rows at all, the null value will be returned.
Alternatively,
an SQL function may be declared to return a set, by specifying the
function's return type as SETOF
. In this case all rows of the
last query's result are returned. Further details appear below.
sometype
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
void
, the last statement must be a SELECT
.
Any collection of commands in the SQL
language can be packaged together and defined as a function.
Besides SELECT
queries, the commands can include data
modification queries (INSERT
,
UPDATE
, and DELETE
), as well as
other SQL commands. (The only exception is that you can't put
BEGIN
, COMMIT
, ROLLBACK
, or
SAVEPOINT
commands into a SQL function.)
However, the final command
must be a SELECT
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 void
.
In that case, the function body must not end with a SELECT
.
For example, this function removes rows with negative salaries from
the emp
table:
CREATE FUNCTION clean_emp() RETURNS void AS ' DELETE FROM emp WHERE salary < 0; ' LANGUAGE SQL; SELECT clean_emp(); clean_emp ----------- (1 row)
The syntax of the CREATE FUNCTION
command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see Section 4.1.2.2, “Dollar-Quoted String Constants”) for the string constant.
If you choose to use regular single-quoted string constant syntax,
you must double single quote marks ('
) and backslashes
(\
) (assuming escape string syntax) in the body of
the function (see Section 4.1.2.1, “String Constants”).
Arguments to the SQL function are referenced in the function
body using the syntax $
: n
$1
refers to the first argument, $2
to the second, and so on.
If an argument is of a composite type, then the dot notation,
e.g., $1.name
, 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:
INSERT INTO mytable VALUES ($1);
but this will not work:
INSERT INTO $1 VALUES (42);
The simplest possible SQL function has no arguments and
simply returns a base type, such as integer
:
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
Notice that we defined a column alias within the function body for the result of the function
(with the name result
), but this column alias is not visible
outside the function. Hence, the result is labeled one
instead of result
.
It is almost as easy to define SQL functions
that take base types as arguments. In the example below, notice
how we refer to the arguments within the function as $1
and $2
.
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$ SELECT $1 + $2; $$ LANGUAGE SQL; SELECT add_em(1, 2) AS answer; answer -------- 3
Here is a more useful function, which might be used to debit a bank account:
CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS $$ UPDATE bank SET balance = balance - $2 WHERE accountno = $1; SELECT 1; $$ LANGUAGE SQL;
A user could execute this function to debit account 17 by $100.00 as follows:
SELECT tf1(17, 100.0);
In practice one would probably like a more useful result from the function than a constant 1, so a more likely definition is
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;
which adjusts the balance and returns the new balance.
When writing functions with arguments of composite
types, we must not only specify which
argument we want (as we did above with $1
and $2
) but
also the desired attribute (field) of that argument. For example,
suppose that
emp
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 double_salary
that computes what someone's
salary would be if it were doubled:
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
Notice the use of the syntax $1.salary
to select one field of the argument row value. Also notice
how the calling SELECT
command uses *
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:
SELECT name, double_salary(emp) AS dream FROM emp WHERE emp.cubicle ~= point '(2,1)';
but this usage is deprecated since it's easy to get confused.
Sometimes it is handy to construct a composite argument value
on-the-fly. This can be done with the ROW
construct.
For example, we could adjust the data being passed to the function:
SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream FROM emp;
It is also possible to build a function that returns a composite type.
This is an example of a function
that returns a single emp
row:
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;
In this example we have specified each of the attributes with a constant value, but any computation could have been substituted for these constants.
Note two important things about defining the function:
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.)
You must typecast the expressions to match the definition of the composite type, or you will get errors like this:
ERROR: function declared to return emp returns varchar instead of text at column 1
A different way to define the same function is:
CREATE FUNCTION new_emp() RETURNS emp AS $$ SELECT ROW('None', 1000.0, 25, '(2,2)')::emp; $$ LANGUAGE SQL;
Here we wrote a SELECT
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
— for example, if we need to compute the result by calling
another function that returns the desired composite value.
We could call this function directly in either of two ways:
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)
The second way is described more fully in Section 33.4.4, “SQL Functions as Table Sources”.
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:
SELECT (new_emp()).name; name ------ None
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:
SELECT new_emp().name; ERROR: syntax error at or near "." at character 17 LINE 1: SELECT new_emp().name; ^
Another option is to use
functional notation for extracting an attribute. The simple way
to explain this is that we can use the
notations attribute(table)
and table.attribute
interchangeably.
SELECT name(new_emp()); name ------ None
-- 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 Andy
The equivalence between functional notation and attribute notation
makes it possible to use functions on composite types to emulate
“computed fields”.
For example, using the previous definition
for double_salary(emp)
, we can write
SELECT emp.name, emp.double_salary FROM emp;
An application using this wouldn't need to be directly aware that
double_salary
isn't a real column of the table.
(You can also emulate computed fields with views.)
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:
CREATE FUNCTION getname(emp) RETURNS text AS $$ SELECT $1.name; $$ LANGUAGE SQL; SELECT getname(new_emp()); getname --------- None (1 row)
Still another way to use a function that returns a composite type is to call it as a table function, as described in Section 33.4.4, “SQL Functions as Table Sources”.
An alternative way of describing a function's results is to define it with output parameters, as in this example:
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)
This is not essentially different from the version of add_em
shown in Section 33.4.1, “SQL Functions on Base Types”. The real value of
output parameters is that they provide a convenient way of defining
functions that return several columns. For example,
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)
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
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;
but not having to bother with the separate composite type definition is often handy.
Notice that output parameters are not included in the calling argument list when invoking such a function from SQL. This is because PostgreSQL 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
DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int); DROP FUNCTION sum_n_product (int, int);
Parameters can be marked as IN
(the default),
OUT
, or INOUT
. An INOUT
parameter serves as both an input parameter (part of the calling
argument list) and an output parameter (part of the result record type).
All SQL functions may be used in the FROM
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.
Here is an example:
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)
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.
Note that we only got one row out of the function. This is because
we did not use SETOF
. That is described in the next section.
When an SQL function is declared as returning SETOF
, the function's final
sometype
SELECT
query is executed to completion, and each row it
outputs is returned as an element of the result set.
This feature is normally used when calling the function in the FROM
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 foo
has the same contents as above, and we say:
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$ SELECT * FROM foo WHERE fooid = $1; $$ LANGUAGE SQL; SELECT * FROM getfoo(1) AS t1;
Then we would get:
fooid | foosubid | fooname -------+----------+--------- 1 | 1 | Joe 1 | 2 | Ed (2 rows)
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:
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)
In the last SELECT
,
notice that no output row appears for Child2
, Child3
, etc.
This happens because listchildren
returns an empty set
for those arguments, so no result rows are generated.
SQL functions may be declared to accept and
return the polymorphic types anyelement
and
anyarray
. See Section 33.2.5, “Polymorphic Types” for a more detailed
explanation of polymorphic functions. Here is a polymorphic
function make_array
that builds up an array
from two arbitrary data type elements:
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)
Notice the use of the typecast 'a'::text
to specify that the argument is of type text
. This is
required if the argument is just a string literal, since otherwise
it would be treated as type
unknown
, and array of unknown
is not a valid
type.
Without the typecast, you will get errors like this:
ERROR: could not determine "anyarray"/"anyelement" type because input has type "unknown"
It is permitted to have polymorphic arguments with a fixed return type, but the converse is not. For example:
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.
Polymorphism can be used with functions that have output arguments. For example:
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)