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CHAPTER 7 UNDERSTANDING T-SQL PROGRAMMING LOGIC
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--3 BEGIN TRY PRINT 1/0; END TRY BEGIN CATCH IF ERROR_NUMBER() = 8134 BEGIN RAISERROR('A bad math error!',16,1); END; END CATCH; Figure 7-18 shows the results. You can provide either a message number or a message string for the message parameter. Batch 1 sets up a custom error message that you can use later when raising an error as in Batch 2. Batch 3 returns a different error to the client than the one that actually happened. Since the code returned an ad hoc error message, the database engine supplied the default number, 50000. The second parameter, severity, ranges from 1 to 25. When under 11, the message is a warning or information. You might want to build a dynamic error based on what happened in your code. If you would like to do this, make sure you save the message in a variable. You cannot build the message dynamically inside the RAISERROR function. See the Books Online article Database Engine Error Severities to learn more about error severities, but you will generally use 16 for errors correctable by the user. The state parameter is an integer between 1 and 255. You can use state to define where in the code the error occurred.
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Figure 7-18. The results of using RAISERROR
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Using TRY CATCH with Transactions
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You can use TRY CATCH to make sure that transactions complete successfully so that the transaction may be rolled back if necessary. Include the transaction in the TRY block. Type in and execute Listing 7-19, which shows a simple example.
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CHAPTER 7 UNDERSTANDING T-SQL PROGRAMMING LOGIC
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Listing 7-19. Using TRY CATCH with a Transaction --1 CREATE TABLE #Test (ID INT NOT NULL PRIMARY KEY); GO --2 BEGIN TRY --2.1 BEGIN TRAN --2.1.1 INSERT INTO #Test (ID) VALUES (1),(2),(3); --2.1.2 UPDATE #Test SET ID = 2 WHERE ID = 1; --2.2 COMMIT END TRY --3 BEGIN CATCH --3.1 PRINT ERROR_MESSAGE(); --3.2 PRINT 'Rolling back transaction'; ROLLBACK; END CATCH; Figure 7-19 shows the error message and the transaction rolled back. Statement 2.1.2 attempts to set the value ID to 2 in the row where it equals 1. This violates the primary key; you cannot have two rows with the value 2. If the entire transaction had been successful, the COMMIT statement would have committed the transaction. Instead, the CATCH block fired, giving you the chance to handle the error.
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Figure 7-19. The results of using TRY CATCH with a transaction
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CHAPTER 7 UNDERSTANDING T-SQL PROGRAMMING LOGIC
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Trapping and handling errors is a very important part of T-SQL. If anything can go wrong, it often will. Practice what you have learned by completing Exercise 7-4.
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Exercise 7-4
Use AdventureWorks2008 to complete this exercise. You can find the solutions in the appendix. 1. Write a statement that attempts to insert a duplicate row into the HumanResources.Department table. Use the @@ERROR function to display the error. 2. Change the code you wrote in question 1 to use TRY CATCH. Display the error number, message, and severity. 3. Change the code you wrote in question 2 to raise a custom error message instead of the actual error message.
Temporary Tables and Table Variables
Temporary, or temp, tables and table variables allow you to save data in short-lived table structures that you can use in your scripts. For example, you may need to save the results of complicated calculations for further processing. The use of temp tables and table variables is controversial. You can find many articles and newsgroup discussions stating that no one should ever use these structures. In my opinion, temp tables and table variables are just more tools that you can use if you need them. I have found that they often allow me to break extremely complicated queries into smaller, more manageable pieces, sometimes with better performance.
Creating Local Temp Tables
Temp tables look and behave just like regular tables except that they live in the tempdb database instead of a user database like AdventureWorks2008. The tempdb database is one of the system databases required for SQL Server. SQL Server also uses tempdb as a work area for sorting and other behind-thescene tasks. To create a local temp table, preface the table name with the number sign (#). Only the connection in which the table was created can see a local temp table. 8 covers creating tables with the CREATE TABLE command, but you have learned how to use the SELECT INTO syntax to create a table. You have also typed in numerous CREATE TABLE statements to create work tables for examples and exercises in 6. Here is the minimum syntax to create a local temp table using the CREATE TABLE command: CREATE TABLE #tableName (<col1> <data type>,<col2> <data type>) Temp tables can have anything that a regular table has such as primary keys, defaults, and indexes. Type in and execute the code from Listing 7-20 to learn how to create a temp table.
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