Database : SQL Server Exception Handling by TRY…CATCH

Like C#, SQL Server also has an exception model to handle exceptions and errors that occurs in T-SQL statements. To handle exception in Sql Server we have TRY..CATCH blocks. We put T-SQL statements in TRY block and to handle exception we write code in CATCH block. If there is an error in code within TRY block then the control will automatically jump to the corresponding CATCH blocks. In Sql Server, against a Try block we can have only one CATCH block.

TRY..CATCH Syntax

 BEGIN TRY
--T-SQL statements
--or T-SQL statement blocks
END TRY
BEGIN CATCH
--T-SQL statements
--or T-SQL statement blocks
END CATCH 

Error Functions used within CATCH block

1. ERROR_NUMBER()

   This returns the error number and its value is same as for @@ERROR function.

2. ERROR_LINE()

   This returns the line number of T-SQL statement that caused error.

3. ERROR_SEVERITY()

   This returns the severity level of the error.

4. ERROR_STATE()

   This returns the state number of the error.

5. ERROR_PROCEDURE()

   This returns the name of the stored procedure or trigger where the error occurred.

6. ERROR_MESSAGE()

   This returns the full text of error message. The text includes the values supplied for any substitutable parameters, such as lengths, object names, or times.

Exception handling example

 BEGIN TRY
DECLARE @num INT, @msg varchar(200)
---- Divide by zero to generate Error
SET @num = 5/0
PRINT 'This will not execute'
END TRY
BEGIN CATCH
PRINT 'Error occured that is'
set @msg=(SELECT ERROR_MESSAGE())
print @msg;
END CATCH
GO 

 BEGIN TRY
DECLARE @num INT
---- Divide by zero to generate Error
SET @num = 5/0
PRINT 'This will not execute'
END TRY
BEGIN CATCH
SELECT ERROR_NUMBER() AS ErrorNumber, ERROR_SEVERITY() AS ErrorSeverity, ERROR_STATE() AS ErrorState, ERROR_PROCEDURE() AS ErrorProcedure, ERROR_LINE() AS ErrorLine, ERROR_MESSAGE() AS ErrorMessage;
END CATCH;
GO 

Note

1. A TRY..CATCH block combination catches all the errors that have a severity between 11 and 19.
2. The CATCH block is executed only if there is an error occurs in T-SQL statements within TRY block otherwise the CATCH block is ignored.
3. Each TRY block is associated with only one CATCH block and vice versa
4. TRY and CATCH blocks can’t be separated with the GO statement. We need to put both TRY and CATCH blocks within the same batch.
5. TRY..CATCH blocks can be used with transactions. We check the number of open transactions by using @@TRANCOUNT function in Sql Server.
6. XACT_STATE function within the TRY..CATCH block can be used to check whether a open transaction is committed or not. It will return -1 if transaction is not committed else returns 1.