SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list, which must be a
SELECT.
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, NULL will be returned.
Alternatively, an SQL function may be declared to return a set,
by specifying the function's return type
as SETOF sometype. In this case
all rows of the last query's result are returned. Further details
appear below.
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 CREATE FUNCTION command requires the body of the
function to be enclosed in single quotes, single quote marks
(') used
in the body of the function must be escaped, by writing two single
quotes ('') or a backslash (\') where each
quote is desired.
Arguments to the SQL function may be 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.emp, may be used to access attributes
of the argument.
To illustrate a simple SQL function, consider the following,
which might be used to debit a bank account:
CREATE FUNCTION tp1 (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 tp1(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 tp1 (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.
Any collection of commands in the SQL
language can be packaged together and defined as a function.
The commands can include data modification (i.e.,
INSERT, UPDATE, and
DELETE) as well
as SELECT queries. 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 it must not end with a SELECT.
For example:
CREATE FUNCTION clean_EMP () RETURNS void AS '
DELETE FROM EMP
WHERE EMP.salary <= 0;
' LANGUAGE SQL;
SELECT clean_EMP();
clean_emp
-----------
(1 row)
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;
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
When specifying 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 attributes 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 your
salary would be if it were doubled:
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
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 a table name to denote
the entire current row of that table as a composite value.
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 AS salary,
25 AS age,
point ''(2,2)'' AS cubicle;
' LANGUAGE SQL;
In this case we have specified each of the attributes
with a constant value, but any computation or expression
could have been substituted for these constants.
Note two important things about defining the function:
The target list order 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 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. For example,
SELECT (new_emp()).name;
name
------
None
We need the extra parentheses to keep the parser from getting confused:
SELECT new_emp().name;
ERROR: parser: parse error at or near "."
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
Another way to use a function returning a row result is to declare a
second function accepting a row type parameter, and pass the function
result to it:
CREATE FUNCTION getname(emp) RETURNS text AS
'SELECT $1.name;'
LANGUAGE SQL;
SELECT getname(new_emp());
getname
---------
None
(1 row)
A table function is one that may be used in the FROM
clause of a query. All SQL language functions may be used in this manner,
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 column
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
(2 rows)
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 say SETOF.
When an SQL function is declared as returning SETOF
sometype, the function's final
SELECT query is executed to completion, and each row it
outputs is returned as an element of the set.
This feature is normally used by calling the function as a table
function. 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;
fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)
Currently, functions returning sets may also be called in the target list
of a SELECT query. For each row that the SELECT
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
target 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 inputs, so no output rows are generated.
