Try Catch Template

Below is an example of an SQL Try Catch template:

BEGIN TRY 

	BEGIN TRANSACTION
	
	-- put the query here	
	
	COMMIT TRANSACTION
	
END TRY
BEGIN CATCH
	IF @@TRANCOUNT > 0
        ROLLBACK TRANSACTION;
    
    DECLARE @ErrorMessage NVARCHAR(4000), @ErrorSeverity INT, @ErrorState INT
    SELECT @ErrorMessage = ERROR_MESSAGE(), @ErrorSeverity = ERROR_SEVERITY(), @ErrorState = ERROR_STATE()
    
    RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState)
END CATCH

Note that the script in the Catch block checks if @@TRANCOUNT > 0 before doing ROLLBACK TRANSACTION. This means, only do rollback when there's at least one active transaction on the current connection. In other word, if it exists at least one BEGIN TRANSACTION that has not been committed yet (by using COMMIT TRANSACTION) on the current connection.

For more information about @@TRANCOUNT: http://msdn.microsoft.com/en-us/library/ms187967.aspx