Query Language (SQL) Functions

PostgreSQL 7.2 Documentation
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# 12.2. Query Language (SQL) Functions

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.

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.

## 12.2.1. Examples

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

## 12.2.2. SQL Functions on Base Types

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

## 12.2.3. SQL Functions on Composite Types

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)

## 12.2.4. SQL Functions Returning Sets

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.

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