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A TRY. . .CATCH block does not trap errors that cause the connection to be terminated, such as a fatal error or a sysadmin executing the KILL command. You also cannot trap errors that occur due to compilation errors, syntax errors, or nonexistent objects. Therefore, you cannot use a TRY. . .CATCH block to test for an object s existence.
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The following code implements structured error handling for the previous code block:
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--TRY...CATCH TRUNCATE TABLE dbo.mytable
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BEGIN TRY BEGIN TRAN INSERT INTO dbo.mytable VALUES(1) INSERT INTO dbo.mytable VALUES(1) INSERT INTO dbo.mytable VALUES(2)
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Lesson 1: Stored Procedures
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COMMIT TRAN END TRY
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BEGIN CATCH ROLLBACK TRAN PRINT 'Catch' END CATCH
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SELECT * FROM dbo.mytable
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One of the more important aspects of a TRY. . .CATCH block is that no error messages are sent to an application unless a RAISERROR is executed within the CATCH block. Within the CATCH block, you have access to the following functions:
n n n n n
ERROR_NUMBER() ERROR_MESSAGE() ERROR_SEVERITY() ERROR_STATE()
The error number of the error thrown The text of the error message The severity level of the error message The function, trigger, or procedure name that was executing
The state of the error
ERROR_PROCEDURE()
when the error occurred
ERROR_LINE()
The line of code within the function, trigger, or procedure that caused
the error
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If you implement a TRY. . .CATCH block, any errors, including system errors, are not returned to the calling application. The only way to return an error message to a calling application is to execute a RAISERROR statement. However, you can only specify a user-defined error message or dynamically construct an error message using RAISERROR. Therefore, you have a slight problem if you want to return a system-generated error to a calling application. If you want to return a system error message, you should dynamically build a message that includes the system error message information, which is returned with a RAISERROR statement that does not supply a message ID. That way, any system-generated messages are always returned with an error number of 50000.
Within the CATCH block, you can determine the current transaction nesting level with the @@TRANCOUNT global variable. You can also retrieve the state of the innermost transaction with the XACT_STATE function. The XACT_STATE function can return the following values:
n n n
1 0 1
An open transaction exists that can be either committed or rolled back. There is no open transaction.
An open transaction exists, but it is in a doomed state. Due to the type of error that was raised, the transaction can only be rolled back.
XACT_ABORT behaves differently when used in conjunction with a TRY block. Instead of terminating the transaction, control is transferred to the CATCH block. However, if
156 CHAPTER 5 Programming Microsoft SQL Server with T-SQL User-Defined Stored Procedures, Functions, Triggers, and Views
XACT_ABORT is turned on, any error is fatal. The transaction is left in a doomed state and XACT_STATE returns 1. Therefore, you cannot commit a transaction inside a CATCH block if XACT_ABORT is turned on.
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Make sure you understand how TRY. . .CATCH blocks handle errors, as well as how XACT_ABORT behaves within a TRY. . .CATCH block.
Executing Stored Procedures
You access a stored procedure by using an EXEC statement. If a stored procedure does not have any input parameters, the only code required is
EXEC <stored procedure>
If a stored procedure has input parameters, you can pass in the parameters either by name or by position:
--Execute by name EXEC <stored procedure> @parm1=<value>, @parm2=<value>,... --Execute by position EXEC <stored procedure> <value>, <value>,...
Passing parameters to a stored procedure by position results in code that is more compact; however, it is more prone to errors. When parameters are passed to a stored procedure by name, changes in the order of parameters within the procedure do not require changes elsewhere in your applications. Regardless of whether you are passing parameters by position or by name, you need to specify a value for each parameter that does not have a default value.
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