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Database Functions

You can define user-specific database functions. In an SQL statement, you can then use these user-defined database functions in the same way as any other predefined functions.

A database function can contain several SQL statements. A series of control structures are also available for the application programmer. You can program loops or branches, for example, within a database function.

Database functions are specialized database procedures that contain any number of input parameters but only one output parameter. The output parameter represents the result of the database function.

Prerequisites

You require the demo data for the SQL Tutorial.

Start the query tool SQL Studio as database administrator MONA with password RED and log on to the demo database instance DEMODB.

Creating a Database Function

To create a database function, use the CREATE FUNCTION statement.

CREATE FUNCTION hotel.avgprice (zip CHAR(5)) RETURNS FIXED(6,2) AS
    VAR sum FIXED(10,2); price FIXED(6,2);
    hotels INTEGER; avg_price FIXED(6,2);
TRY
  SET sum = 0; SET hotels = 0;
  DECLARE functionresult CURSOR FOR
  SELECT price FROM hotel.room,hotel.hotel WHERE zip = :zip AND
  room.hno = hotel.hno AND type = 'single';
    WHILE $rc = 0 DO BEGIN
      FETCH functionresult INTO :price;
      SET sum = sum + price;
      SET hotels = hotels + 1;
    END;
CATCH
  IF $rc <> 100 THEN STOP ($rc, 'unexpected error');
CLOSE functionresult;
IF hotels > 0 THEN RETURN sum/hotels
  ELSE RETURN NULL;

The avgprice database function determines the average price for single rooms in hotels that are located within the specified zip code range.

Explanation

● The input parameter is the zip code zip; the data type of the output value is specified after the keyword RETURNS.

● The SQL statements (the SELECT statement, for example) and control structures (WHILE statement and an IF statement, for example) that are required for executing the desired function are defined after the keyword AS.

● The variables required within the database function are defined after the keyword VAR.

● Troubleshooting: if an SQL error occurs in the statement list between TRY and CATCH, the system branches directly to the statement that follows CATCH. The actual troubleshooting routine can be programmed in this statement.

● SET can be used to assign values to the sum and hotels variables.

● The DECLARE CURSOR statement is used to define the named result table functionresult .

● The tables in the SELECT statement must always be specified completely, that is, with the owner specified. Note that the zip code is transferred as the parameter: :zip.

● The WHILE statement enables statements to be repeated in response to certain conditions. The statement is executed as long as the search condition specified is met.

● The FETCH statement assigns the values from the current row of the result table functionresult to parameters.

● The $rc variable returns a numeric error code after the SELECT statement has been executed. The value 0 means that the SELECT statement was successfully executed.

● An IF statement first evaluates the search condition. If this is met, the statement specified in the THEN branch is executed. Otherwise, the statement in the ELSE branch (if defined) is executed.

● The CLOSE statement deletes the result table functionresult.

● The output parameter is defined after the keyword RETURN.