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SQL: A Beginner s Guide
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each applicable data modification statement that is executed, no matter how many rows are affected. If you do not include this clause in your trigger definition, the STATEMENT option is assumed, and the trigger fires only once for each statement. Next in the syntax is the optional WHEN clause. The WHEN clause allows you to define a search condition that limits the scope of when the trigger is invoked. The WHEN clause is similar to the WHERE clause of a SELECT statement. You specify one or more predicates that define a search condition. If the WHEN clause evaluates to true, the trigger fires; otherwise, no trigger action is taken. However, this doesn t affect the initial data modification statement that was executed against the subject table; only the triggered SQL statements defined in the trigger definition are affected. Finally, the last component that your CREATE TRIGGER statement must include is one or more SQL statements (sometimes called the trigger body) that are executed when the trigger is invoked and, if a WHEN clause is included, that clause evaluates to true. If the trigger definition includes more than one triggered SQL statement, or if you are using Oracle, those statements must be enclosed in a BEGIN...END block, like those you saw in 13. However, there is one difference from what you saw before. When used in a trigger definition, the BEGIN keyword must be followed by the ATOMIC keyword to notify the SQL implementation that the statements within the block must be handled as a unit. In other words, either all the statements must be executed successfully, or none of the results of any statement executions can persist. Without the ATOMIC keyword, it would be possible for some statements to be executed while others fail to be executed.
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Many implementations do not support the use of the ATOMIC keyword in the BEGIN...END block of the triggered SQL statements. This includes both SQL Server and Oracle. Also, with Oracle, all trigger and procedure bodies must be enclosed in BEGIN END blocks.
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Aside from the issue of the ATOMIC keyword, the triggered SQL statements, including the BEGIN...END block, can consist of almost any SQL statements, depending on the limitations of your SQL implementation. Be sure to check the product documentation to determine what limitations might be placed on the triggered SQL statements and how triggers are generally created and implemented.
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Now let s return to the REFERENCING clause of the CREATE TRIGGER statement. The purpose of this clause is to allow you to define correlation names for the rows stored in the transition tables or for the transition tables as a whole. As you ll recall from the Understand SQL Triggers section earlier in this chapter, the transition tables hold the data that has been updated, inserted, or deleted in the subject table. The correlation names, or aliases, can then be used in the triggered SQL statements to refer back to the data that is being held in the transition tables. This can be particularly handy when trying to modify data in a second table based on the data modified in the subject table. (This will be made clearer when we look at examples later in the chapter.)
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Creating SQL Triggers
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If you refer back to the syntax in the previous section, you ll notice that the optional REFERENCING clause includes the <alias options> placeholder. SQL supports four options for this clause:
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REFERENCING OLD [ROW] [AS] <alias> REFERENCING NEW [ROW] [AS] <alias> REFERENCING OLD TABLE [AS] <alias> REFERENCING NEW TABLE [AS] <alias>
Notice that, in the first two options, the ROW keyword is not mandatory. If you don t specify ROW, it is assumed. Notice too that the AS keyword is optional in all cases. However, for the purposes of maintaining clear, self-referencing code, I recommend that you use the complete option whenever you include it in a trigger definition. Depending on the type of trigger (update, insert, or delete) and the FOR EACH option (ROW or STATEMENT), you can include up to four REFERENCING options in your trigger definition, one of each type. However, you cannot include more than one of any single type. For example, you cannot include two OLD ROW options in your trigger definition. When adding REFERENCING options to your trigger definition, you must follow these guidelines:
You cannot use the NEW ROW and NEW TABLE options for delete triggers because no new data is created. You cannot use the OLD ROW and OLD TABLE options for insert triggers because no old data exists. You can use all four options in an update trigger because there is old data and new data when you update a table. You can use the OLD ROW and NEW ROW options only when you specify the FOR EACH ROW clause in the trigger definition.
Once you define your REFERENCING clauses and assign the appropriate aliases, you re ready to use those aliases in your triggered SQL statements, in the same way you used correlation names in your SELECT statements.
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