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Database Processing and Stored Procedures
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need to actually execute the routine; it can simply return the same results that it returned the last time. Another form of hint tells the DBMS whether an external stored procedure or function reads database contents and whether it modifies database contents. This not only allows the DBMS to optimize database access, but can also impose a security restriction on external routines from other sources. Other hints determine whether a SQL-function should be called if one of its parameters has a NULL value, and control how the DBMS selects the specific function or procedure to be executed when overloading is used.
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Triggers were addressed for standardization as part of the SQL3 effort, which led to the eventual publication of the SQL:1999 ANSI/ISO standard. By that time, many commercial DBMS products had already implemented triggers, and the standard synthesized the specific capabilities that had proven useful in practice. Like the commercial products, ANSI/ISO standard triggers are defined for a single, specific table. The standard permits trigger definitions only on tables, not on views. The proprietary SQL Server, Oracle and Informix trigger mechanisms shown in the examples throughout this chapter provide a context for examining the ANSI/ISO standard mechanism. The standard does not provide any radical departure from the capabilities already described for the various DBMS products. Here is how the standard compares to them: I Naming. The standard treats triggers as named objects within the database. I Types. The standard provides INSERT, DELETE, and UPDATE triggers; UPDATE triggers can be associated with the update of a specific column or group of columns. I Timing. The standard provides for triggers that operate before a database update statement or after the statement. I Row-level or statement-level operation. The standard provides for both statement-level triggers (executed once per database-updating statement) and row-level triggers (executed repeatedly for each row of the table that is modified). I Aliases. Access to the before and after values in a modified row or table is provided via an alias mechanism, like the table aliases used in the FROM clause. You use the SQL:1999 CREATE TRIGGER statement, shown in Figure 20-22, to define a trigger. The statement clauses are familiar from the proprietary trigger examples throughout the earlier sections of this chapter. One very useful extension provided by the standard is the WHEN clause that can be specified as part of a triggered action. The WHEN clause is optional, and it operates like a WHERE clause for determining whether a triggered action will be carried out. When
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Figure 20-22.
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The 1999 CREATE TRIGGER syntax diagram
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DBMS executes the particular type of statement specified in the trigger definition, it evaluates the search condition specified in the WHEN clause. The form of the search condition is exactly like the search condition in a WHERE clause, and it will produce either a TRUE or FALSE result. The triggered action is carried out only if the result is TRUE. To provide security for triggers, the SQL:1999 standard establishes a new TRIGGER privilege that may be granted for specific tables to specific users. With this privilege, a user may establish a trigger on the table. The owner of a table is always allowed to establish triggers on the table.
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Database Processing and Stored Procedures
Summary
Stored procedures and triggers are two very useful capabilities for SQL databases used in transaction-processing applications: I Stored procedures allow you to predefine common database operations, and invoke them simply by calling the stored procedure, for improved efficiency and less chance of error. I Extensions to the basic SQL language give stored procedures the features normally found in programming languages. These features include local variables, conditional processing, branching, and special statements for row-by-row query results processing. I Stored functions are a special form of stored procedure that return a value. I Triggers are procedures whose execution is automatically initiated based on attempted modifications to a table. A trigger can be fired by an INSERT, DELETE, or UPDATE statement for the table. I There is wide variation in the specific SQL dialects used by the major DBMS brands to support stored procedures and triggers. I There is now an international standard for stored procedures (but not triggers); as one of the newer standards, it has not yet had a major impact on the actual implementation by leading DBMS vendors.
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