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 multi-row 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.
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.
CREATE FUNCTION clean_EMP () RETURNS integer AS '
DELETE FROM EMP
WHERE EMP.salary <= 0;
SELECT 1 AS ignore_this;
' LANGUAGE SQL;
SELECT clean_EMP();
x --- 1
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. (However, as we'll see below, there are some unfortunate restrictions on how the function may be used.) 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.
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
In the present release of PostgreSQL there are some unpleasant restrictions on how functions returning composite types can be used. Briefly, when calling a function that returns a row, we cannot retrieve the entire row. We must either project a single attribute out of the row or pass the entire row into another function. (Trying to display the entire row value will yield a meaningless number.) For example,
SELECT name(new_emp());
name ------ None
This example makes use of the function notation for projecting attributes. The simple way to explain this is that we can usually use the notations attribute(table) and table.attribute interchangeably:
--
-- 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
The reason why, in general, we must use the function syntax for projecting attributes of function return values is that the parser just doesn't understand the dot syntax for projection when combined with function calls.
SELECT new_emp().name AS nobody; ERROR: parser: parse error at or near "."
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)
As previously mentioned, an SQL function may be declared as returning SETOF sometype. In this case the function's final SELECT query is executed to completion, and each row it outputs is returned as an element of the set.
Functions returning sets may only 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. An example:
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.