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14. What is the difference between a conditional statement and a compound statement 15. What are two types of looping statements A BEGIN...END B IF...END IF C LOOP...END LOOP D WHILE...END WHILE 16. Which type of parameter can return a value when you invoke a procedure 17. What step must you take when calling a procedure that includes an output parameter 18. How does a CREATE FUNCTION statement differ from a CREATE PROCEDURE
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19. You re calling a procedure named GET_TOTALS. The procedure does not include any
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parameters, but does include a SELECT statement that queries the CD_INVENTORY table. What SQL statement should you use to invoke this parameter
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20. You create a procedure named GET_CD_INFO that selects data about an artist from
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the CD_INFO table. The procedure includes one input parameter. You want to call that procedure with the value Bonnie Raitt. What SQL statement should you use to invoke the procedure
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21. What are two types of schema objects that you can use to store a SELECT statement
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Creating SQL Triggers
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SQL: A Beginner s Guide
Key Skills & Concepts
Understand SQL Triggers Create SQL Triggers Create Insert Triggers Create Update Triggers Create Delete Triggers
p to this point in the book, you have learned to create a number of schema objects that you can access or invoke by using SQL statements. For example, you learned how to create tables, views, and SQL-invoked routines. In each case, once you create these objects, you need to take some sort of action to interact directly with them, such as executing a SELECT statement to retrieve data from a table or using a CALL statement to invoke a procedure. However, SQL supports objects that perform actions automatically. These schema objects, which are known as triggers, respond to modifications made to data within a table. If a specified modification is made, the trigger is automatically invoked, or fired, causing an additional action to occur. As a result, you never directly invoke the trigger taking an action defined in the trigger implicitly causes the invocation. In this chapter, we ll explore triggers and how they re used when table data is modified. We ll also look at examples of how to create the three basic types of triggers insert, update, and delete and how they can be defined to extend your database s functionality and help to ensure the integrity of the data.
Understand SQL Triggers
If you ve worked around any SQL products before, you ve no doubt seen triggers implemented in one of your organization s databases, or at least heard the term tossed about. Most relational database management systems (RDBMSs) implemented triggers in their products long ago, although it wasn t until SQL:1999 that triggers were added to the standard. The result of the products preceding the standard is that trigger implementations are very proprietary among the SQL products, and thus support different types of functionality and are implemented in different ways. For example, MySQL 5.0 does not support triggers, but the additional feature is promised for version 5.1. On the other hand, SQL Server and Oracle currently support triggers, but SQL Server triggers are somewhat limited in scope, compared to the SQL standard, whereas Oracle triggers are more robust yet neither product implements triggers according to the specifications of the SQL standard. Despite this, there are a number of similarities among the products (such as the use of a CREATE TRIGGER statement to create a trigger), and the implementations of triggers in the various products share some basic characteristics, particularly that of being able to fire automatically to perform an action secondary to the primary action that invoked the trigger.
14:
Creating SQL Triggers
NOTE
The functionality supported by triggers is sometimes referred to as active database. In fact, this term is used to describe one of the optional packages that are included in the SQL standard. The package PKG008 defines how triggers are implemented in SQL. (A package is a set of features to which a product can claim conformance in addition to Core SQL.) For more information about SQL:2006 conformance, see 1.
Before we get into the specifics of how to implement triggers, let s take a look at the trigger itself, which, as I said, is a schema object (in the same sense as a table, view, or SQLinvoked routine). A trigger definition defines the characteristics of the trigger and what actions are taken when the trigger is invoked. These actions, which are specified in one or more SQL statements (referred to as the triggered SQL statements), can include such events as updating tables, deleting data, invoking procedures, or performing most tasks that you can perform with SQL statements. Any limitations placed on those statements are usually the ones placed by the SQL implementation. Triggers are invoked when you insert data into a table, update data, or delete data. By defining one or more triggers on a table, you can specify which data-modification actions will cause the trigger to fire. The trigger is never invoked unless the specified action is taken. As you can probably conclude from this, SQL supports three types of triggers: insert, update, and delete. Each type corresponds with the applicable data modification statement. For example, an insert trigger is fired when the INSERT statement is executed against the specified table. Although a trigger is a schema object, separate from table objects, it can be associated with only one table, which you specify when you create your trigger definition. When the applicable data modification statement is invoked against that table, the trigger fires; however, it will not fire if a similar statement is invoked against a different table, or if a statement other than the specified type is invoked against the same table. In this sense, a trigger can be thought of as a table object, despite the fact that it is created at the schema level. If a trigger fails, raising an error condition, the SQL statement that caused the trigger to fire also fails and is rolled back. This is how triggers can be used to enforce complex constraints the trigger is written to perform whatever tests are necessary to verify that the constraint conditions are met, and if not, ends by raising an error condition.
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