Database procedures are programs that can be called up from an application program in the same way as an SQL statement. A database procedure 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 procedure.
You can define both input and output parameters in order to transfer individual values to a database procedure or to obtain results.
You can use database procedures, for example, to formulate complex integrity rules that check the permissibility of values. In database procedures, operations can be provided for application objects. Any changes to these rules or operations can be made at a central location, namely in the database procedure. This means that they do not have to be made individually in each application. The programs are more clearly organized and, as a result, more user friendly.
Using database structures also makes it easier to grant privileges. All you have to do is grant the call privilege; you do not need to grant privileges for the addressed database objects.
To create a database procedure, use the CREATE DBPROC statement.
CREATE DBPROC
avg_price (IN zip CHAR(5), OUT avg_price FIXED(6,2)) AS
VAR sum FIXED(10,2); price FIXED(6,2); hotels
INTEGER;
TRY
SET sum = 0; SET hotels = 0;
SELECT price FROM mona.room,mona.hotel WHERE zip = :zip AND
room.hno = hotel.hno AND type = 'single';
WHILE $rc = 0 DO BEGIN
FETCH INTO :price;
SET sum = sum + price;
SET hotels = hotels + 1;
END;
CATCH
IF $rc <> 100 THEN STOP ($rc, 'unexpected error');
IF hotels > 0 THEN SET avg_price = sum / hotels
ELSE STOP (100, 'no hotel found');
The database procedure avg_price determines the average price for single rooms in hotels that are located within the specified zip code range.
· IN zip CHAR(5), OUT avg_price FIXED(6,2) : the input parameter is the zip code zip; the output parameter is the average price avg_price.
· The SQL statements (in this example, a SELECT statement) and control structures (in this example, a WHILE and an IF statement) that are required for executing the desired procedure are defined after the keyword AS.
· The variables required within the database procedure 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 tables in the SELECT statement must always be complete, that is, with the owner specified. Make sure 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 specified search condition is true.
· 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.
· The IF statement first evaluates the search condition. If this is fulfilled, the statement specified in the THEN branch is executed. Otherwise, the statement in the ELSE branch (if defined) is executed.
See also:
CREATE DBPROC Statement (create_dbproc_statement)
To call up a database procedure, use the CALL statement.
If the hotel table is filled with the data specified in HOTELDB_SIMPLE, you can test the following CALL statement:
CALL avg_price (20005, :avg_price)
Result
Out(1) |
135 |
If the mode of a formal parameter in the database procedure is OUT, the corresponding expression must be a parameter specification. The CALL statement contains the parameter specification :avg_price.
See also:
CALL Statement (call_statement)
To drop a database procedure, use the DROP DBPROC statement.
DROP DBPROC avg_price
You can use this SQL statement to drop the database procedure definition.
See also:
DROP DBPROC Statement (drop_dbproc_statement)