642Chapter14•ImplementingObjects trigger to fire occur within the context of a single transaction. It is possible to roll back INSERT, UPDATE, and DELETE statements from within a trigger. This is useful for complex data validation purposes. You can use triggers to manually cascade changes through related tables; to guard against malicious or incorrect insert, update, and delete operations; and to enforce other restrictions that are more complex than those defined by using CHECK constraints. Ex a m Wa r n i n g Triggers should be used sparingly because they have severe performance implications. In addition,, triggers can be difficult to maintain. Unlike CHECK constraints, triggers can reference columns in other tables. For example, a trigger can use a SELECT statement from another table to compare to the inserted or updated data and to perform additional actions, such as modifying the data, or displaying a user-defined error message. Triggers can evaluate the state of a table before and after a data modification and take actions based on that difference. Multiple triggers of the same type (INSERT, UPDATE, or DELETE) on a table allow multiple different actions to take place in response to the same modification statement. Triggers also allow the use of custom error messages. Triggers can be specified as FOR, AFTER, or INSTEAD OF. The trigger action will fire during the DML statement, after the DML statements, or in place of the DML statement, respectively. Triggers can be specified for UPDATE, INSERT, DELETE, or any combination of these. How do you know what data the user is attempting to insert, update, or delete within a trigger? The trigger can access special tables called INSERTED and DELETED. These virtual tables exist only while the trigger is executing. The INSERTED table contains the new values you are attempting to insert into the table, or new values of the row when you are attempting to update data. The DELETED table contains the row you are attempting to delete or old values of the row when you are attempting to update data. Make use of these tables by querying them to determine old and new values of the data being affected. To cancel the DML statement from within a trigger and roll it back, use the ROLLBACK TRANSACTION statement. Example 14.17 demonstrates how to create triggers, and the effect they take after a DML statement is executed on the table to which the trigger is bound. Implementing Objects • Chapter 14 643 Example 14.17 Creating a Trigger on the Stars Table CREATE TABLE StarHistory (StarHistoryId int IDENTITY PRIMARY KEY, StarName varchar(50), OldType ntext, NewType ntext, DateChanged DateTime); GO CREATE TRIGGER UpdateStarHistory on dbo.Stars AFTER INSERT, UPDATE AS BEGIN INSERT StarHistory (StarName, OldType, NewType, DateChanged) SELECT INSERTED.StarName, DELETED.StarType, INSERTED.StarType, GETDATE() FROM INSERTED LEFT JOIN DELETED on INSERTED.StarID = DELETED.StarID END GO UPDATE Stars SET StarType = 'Burnt out' WHERE StarName = 'Sun'; GO SELECT * FROM StarHistory Results: StarHistoryId StarName OldType NewType DateChanged 1 Sun Yellow dwarf Burnt out 2009-01-21 11:56:29.530 Te s T Da y Ti p You don’t need to be able to write a trigger for the exam. Make sure that you understand the concepts behind triggers and why you may wish to use them. Remember that triggers can be defined on views as well. Creating INSTEAD OF triggers on a view that is not updateable will allow you to perform actions when a user attempts to insert, update, or delete data in the view. 644 Chapter14•ImplementingObjects EXERCISE 14.2 Wo r k i n g W i t h Vi e W s a n d st o r e d Pr o C e d u r e s In this exercise, you will use Transact-SQL statements to create views and stored procedures that show and modify data in the Planets table that you created in Exercise 14.1. Make sure that you have completed Exercise 14.1 before proceeding with this exercise. 1. Switch to SQL Server Management Studio. 2. Create a new query against the AdventureWorks database. 3. Create a view named TerrestrialPlanets that shows planet name and planet description for only those planets where the type is “Terrestrial Planet” and insert a new row into the view. View the data in the underlying table to ensure the new row has been inserted. Use the following statement. CREATE VIEW TerrestrialPlanets AS SELECT PlanetName, PlanetDescription FROM Planets WHERE PlanetType = 'Terrestrial Planet' GO SELECT * FROM TerrestrialPlanets; INSERT TerrestrialPlanets VALUES ('Mars', 'Mars is the fourth planet from the Sun in the Solar System.') SELECT * FROM Planets 4. Create a trigger that will update the PlanetType to “Terrestrial Planet” when a new row is inserted into the TerrestrialPlanets view. Use the following statement. DELETE FROM Planets WHERE PlanetName = 'Mars'; GO CREATE TRIGGER UpdatePlanetType on dbo.Planets AFTER INSERT AS ImplementingObjects•Chapter14 645 BEGIN UPDATE Planets SET PlanetType = 'Terrestrial Planet' FROM Planets join INSERTED ON INSERTED.PlanetName = Planets. PlanetName END GO INSERT TerrestrialPlanets VALUES ('Mars', 'Mars is the fourth planet from the Sun in the Solar System.') SELECT * FROM Planets 5. Create and test a stored procedure that will update the radius for a given planet. Use the following statement. CREATE PROCEDURE UpdateRadius @Radius int, @PlanetName varchar(50) AS BEGIN UPDATE Planets SET Radius = @Radius WHERE PlanetName = @ PlanetName END; GO EXECUTE UpdateRadius 3376, 'Mars'; GO SELECT PlanetName, Radius FROM Planets 646 Chapter14•ImplementingObjects Summary of Exam Objectives In this chapter you have learned about creating database objects. As a database administrator, you must understand the types of objects that comprise a database system. Database objects are divided into two broad categories: storage and pro- grammability. Tables store data and are created using the CREATE TABLE state- ment. For each column in the table you must select a built-in or a user-defined data type. Indexes are created on tables to maximize query performance. Constraints are associated with table columns and define the rules to which data in a particular column or columns must adhere. Constraints can also define relationships between tables, like the necessity to have a Customer entity associated with every Order entity. These are known as foreign key constraints. Programmability objects allow you to define Transact-SQL statements that will be reused over and over again. Views are based on Transact-SQL SELECT state- ments. They represent a way of viewing a data set, and they show data from one or more underlying tables. Views based on a single underlying table can be updated. Specifying the WITH SCHEMABINDING option when creating a view prevents the underlying table from being modified, if the modification will affect the view. Stored procedures are compiled Transact-SQL statements that perform particular actions. Stored procedures can accept parameters and return values. Functions are similar to stored procedures, except that they always return a value, and they never update data. Triggers are actions defined on tables that will execute every time data in a table changes. Triggers can be created FOR UPDATE, FOR DELETE, and FOR INSERT. You can also create triggers for AFTER and INSTEAD OF DML operations. Exam Objectives Fast Track Understanding DDL and DML Language Elements Data Definition Language (DDL) contains statements used to add, modify, ˛ and remove objects from the database. The DDL statements are CREATE, ALTER, and DRO P. These statements can be used to create and manipulate tables, data types, indexes, views, stored procedures, functions, and triggers. Data Manipulation Language (DML) is a part of the Transact-SQL lan- ˛ guage that allows you to insert, modify, and delete data in SQL Server tables. The core statements that comprise DML are INSERT, UPDATE, DELETE, and MERGE. DDL manipulates database structure, while DML manipulates actual data ˛ stored in tables. . columns in other tables. For example, a trigger can use a SELECT statement from another table to compare to the inserted or updated data and to perform additional actions, such as modifying the data,. within a trigger? The trigger can access special tables called INSERTED and DELETED. These virtual tables exist only while the trigger is executing. The INSERTED table contains the new values. attempting to insert into the table, or new values of the row when you are attempting to update data. The DELETED table contains the row you are attempting to delete or old values of the row when you