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Before we move on to discussing how a trigger is created, I want to touch on the subject of how triggers are executed, with regard to the trigger execution context, a type of SQL execution context. You can think of an execution context as a space created in memory that holds a statement process during the execution of that statement. SQL supports several types of execution contexts, triggers being one of them. A trigger execution context is created each time a trigger is invoked. If multiple triggers are invoked, an execution context is created for each one. However, only one execution context can be active in a session at any one time. This is important when a trigger in one table causes a trigger in a second table to be fired. Let s take a look at Figure 14-1 to help illustrate this point.
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Table 1
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Trigger execution context for update trigger Update trigger on Table 1 Transition table for old data Transition table for new data
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Trigger execution context for insert trigger Transition table for new data
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Figure 14-1
Trigger execution contexts for two triggers
Notice that the figure contains three tables. An update trigger is defined on Table 1, and an insert trigger is defined on Table 2. When an UPDATE statement is executed against Table 1, the update trigger fires, creating a trigger execution context that becomes active. However, the update trigger, which is defined to insert data into Table 2, invokes the insert trigger on Table 2 when the first trigger attempts to insert data into that table. As a result, a second execution context is created, which becomes the active one. When the second trigger execution has completed, the second execution context is destroyed, and the first execution context becomes active once more. When the first trigger execution has completed, the first trigger execution context is destroyed. A trigger execution context contains the information necessary for the trigger to be executed correctly. This information includes details about the trigger itself and the table on which the trigger was defined, which is referred to as the subject table. In addition, the execution context includes one or two transition tables, as shown in Figure 14-1. The transition tables are virtual tables that hold data that is updated in, inserted into, or deleted from the subject table. If data is updated, then two transition tables are created, one for the old data and one for the new data. If data is inserted, one transition table is created for the new data. If data is deleted, one transition table is created for the old data. The transition tables and some of the other information in the trigger execution context are used by the SQL statements that perform the triggered action. You ll learn more about how this information is used in the following section, when we look at the CREATE TRIGGER syntax.
14:
Creating SQL Triggers
Create SQL Triggers
Now that you have a general overview of triggers, let s take a look at the syntax you use to create them. Most of the syntax is concerned with defining the characteristics of the trigger, such as the name of the trigger and the type. Only at the end of the statement do you define the triggered SQL statements that specify the actions taken by the trigger when it is invoked. The basic syntax for creating a trigger definition is as follows: CREATE TRIGGER <trigger name> { BEFORE | AFTER } { INSERT | DELETE | UPDATE [ OF <column list> ] } ON <table name> [ REFERENCING <alias options> ] [ FOR EACH { ROW | STATEMENT } ] [ WHEN ( <search condition> ) ] <triggered SQL statements> Let s take a look at each line of the syntax. The first line is fairly straightforward. You simply provide a name for the trigger following the CREATE TRIGGER keywords. In the second line, you must designate whether the trigger is invoked before or after the data modification statement is applied to the subject table. For example, if you re defining an insert trigger, you can specify whether the triggered SQL statements are executed before the data is inserted into the subject table (by using the BEFORE keyword) or after the data is inserted into the subject table (by using the AFTER keyword). This feature is particularly useful when one of the tables is configured with a referential integrity constraint and cannot contain data before that data exists in the other table. (For information about referential integrity, see 4.) Depending on the nature of the triggered action that is defined, it may not matter whether you designate BEFORE or AFTER because the triggered action may have no direct relation to the data modified in the subject table. In the third line of syntax, you specify whether the trigger is an insert, delete, or update trigger. If it is an update trigger, you have the option of applying the trigger to one or more specific columns. If more than one column is specified, you must separate the column names with commas. In the next line of syntax, you must specify an ON clause that includes the name of the subject table. This is the table on which the trigger is applied. The trigger can be applied to only one table. Up to this point, all the syntax we ve looked at is required, except for specifying column names in update trigger definitions, which is optional. However, the next several clauses are not mandatory, but they add important capabilities to your trigger. The first of these clauses is the REFERENCING clause. This clause allows you to specify how data that is held in the trigger execution context is referenced within the WHEN clause or the triggered SQL statements. We ll look at the REFERENCING clause in more detail in the following section, Referencing Old and New Values. The next line of syntax contains the FOR EACH clause, which includes two options: ROW or STATEMENT. If you specify ROW, the trigger is invoked each time a row is inserted, updated, or deleted. If you specify STATEMENT, the trigger is invoked only one time for
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