By defining a trigger, you can have SQL statements automatically execute when an operation (updating, insertion, deletion) is performed on a particular table. A trigger defines such specifications as the table to which the trigger applies, the SQL statement that activates the trigger (trigger event SQL), the SQL statements that are to be executed automatically (trigger SQL statements), and conditions for execution of the action (trigger action search conditions). When an SQL statement that satisfies the trigger action search conditions is executed on a table for which a trigger is defined, the trigger SQL statements are automatically executed. Figure 5-6 provides an overview of triggers.

Figure 5-6 Overview of triggers

Explanation

A trigger is defined to provide cumulative updating of data in the product management historical table on price changes that occur in the product management table. In this example, when the trigger event SQL statement (UPDATE in this case) is executed from the UAP, the product number, the price before the change, and the price after the change in the row containing the changed price are added to the product management historical table.

Note, however, that when you define a trigger for a table, you must re-generate any function, procedure, or trigger SQL object that uses that table, because any SQL object that uses that table becomes invalid. Similarly, if you define, change, or delete any resources that are used by the trigger (tables, indexes, and so on), the trigger SQL object becomes invalid, so you must re-generate it as well. For details, see the HiRDB Version 8 Installation and Design Guide.

Organization of this section

(1) Application criteria

(2) Before you define a trigger

(3) Trigger definition statements