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7: Debugging and Error Handling
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not report a syntax error during compilation but will report a runtime error during execution.
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Create Procedure prDeferredNameResolution As set nocount on select 'Start' select * from NonExistingTable select 'Will execution be stopped ' return
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If you attempt to run this stored procedure, SQL Server will return the following:
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----Start Server: Msg 208, Level 16, State 1, Procedure prDeferredNameResolution, Line 7 Invalid object name 'NonExistingTable'.
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The execution will be stopped. Even an error handler written in Transact-SQL will not be able to proceed at this point.
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Be very cautious when you use cursors: test the status after each fetch; place error handling after each command; do not forget to close and deallocate the cursor when you do not need it any more. There are many rules and regulations for using cursors, and some of them might seem trivial, but even the smallest mistake can halt the execution of your code.
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The overconfidence that comes with routine may be your worst enemy. If you perform the same or similar tasks over and over again, you can lose focus and skip basic steps. Do not put code into production before it is thoroughly tested; do not place bug fixes directly into production; use error handling even if the code seems straightforward and the chance for error slight.
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Error Handling
A developer s effective use of error handling procedures is often an excellent indicator of his or her experience in that particular programming language. Those of
SQL Server 2000 Stored Procedure & XML Programming
us who deal with a C or Visual Basic environment are accustomed to a whole set of feature-rich error handling objects, procedures, and functions. Compared with those, T-SQL seems rather inadequate. You can employ only one function and a few procedures for setting or raising errors. However, the apparent limitations of the tool set still do not justify sloppy solutions. This section starts by investigating how errors can be returned to a caller. Then, it discusses the concept of error handling and offers a comprehensive methodology for implementation. It also discusses some alternative techniques involving the Set Xact_Abort On statement.
Raiserror
An important tool for implementing error handling is the Raiserror statement. Its main purpose is to return a user-defined or system-defined message to the caller. Open Query Analyzer and execute the following statement:
Raiserror ('Error occurred!', 0, 1)
The server will display an error message in the Result pane (see Figure 7-8). The second and third parameters indicate the severity and state of the error.
Figure 7-8
Using Raiserror
7: Debugging and Error Handling
Naturally, this statement does more than return this meager result. It also sets the value of the @@error function to the number of the error that you have raised. If you do not specify an error number (as was the case in the previous example), SQL Server will assign the default of 50000. You can also display errors that are predefined in SQL Server if you reference them by their numbers, and you can define your own errors using the sp_addmessage system stored procedure:
Exec sp_addmessage 50001, 16, 'Unable to update Total of LeaseSchedule'
Then you can display this message using the following statement:
Raiserror (50001, 16, 1)
The server will return the following:
Server: Msg 50001, Level 16, State 1, Line 1 Unable to update Total of LeaseSchedule
You can set the state and severity of the error, record the error in the SQL Server Error Log, and even record the error in the Windows NT Error Log:
Raiserror (50001, 16, 1) WITH LOG
Enterprise Manager contains a tool for displaying and editing error messages. To start it, click a server node and then choose Tools | Manage SQL Server Messages.
SQL Server 2000 Stored Procedure & XML Programming
Using Error Handling
Since T-SQL is so laconic (critics may say feature poor), development DBAs commonly express themselves in a very concise manner. DBAs frequently write ad hoc scripts for one-time use or manual execution, and they thus neglect the need for consistent error handling. Logic that is fine in standard languages like Visual Basic or C frequently does not work in T-SQL. For example, an error may occur in T-SQL, but if T-SQL does not consider it fatal, processing will continue. Also, if the error is fatal, all processing will stop. The process does not react: it is just killed.
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