Handling SQL Server Errors
By Bill Graziano
on 5 April 2010
| 10 Comments
| Tags: Stored Procedures, SQL Server 2005, Transactions, SQL Server 2008 Features, Error Handling
This article covers the basics of TRY CATCH error handling in T-SQL introduced in SQL Server 2005. It includes the usage of common functions to return information about the error and using the TRY CATCH block in stored procedures and transactions.
SQL Server uses the following syntax to capture errors in Transact-SQL
statements:
BEGIN TRY
SELECT [First] = 1
SELECT [Second] = 1/0
SELECT [Third] = 3
END TRY
BEGIN CATCH
PRINT 'An error occurred'
END CATCH
This returns the following output:
First
-----------
1
(1 row(s) affected)
Second
-----------
(0 row(s) affected)
An error occurred
A key difference from SQL Server 2000 is that execution is halted when
SQL Server encounters an error. At that point execution transfers to
the CATCH block. This error isn't returned to the client application
or calling program. The TRY CATCH block consumes the error.
Error Functions
Inside the CATCH block there are a number of specialized functions to
return information about the error.
BEGIN TRY
SELECT [Second] = 1/0
END TRY
BEGIN CATCH
SELECT [Error_Line] = ERROR_LINE(),
[Error_Number] = ERROR_NUMBER(),
[Error_Severity] = ERROR_SEVERITY(),
[Error_State] = ERROR_STATE()
SELECT [Error_Message] = ERROR_MESSAGE()
END CATCH
Second
-----------
(0 row(s) affected)
Error_Line Error_Number Error_Severity Error_State
----------- ------------ -------------- -----------
2 8134 16 1
(1 row(s) affected)
Error_Message
---------------------------------------------------
Divide by zero error encountered.
(1 row(s) affected)
These are the functions you can use inside a CATCH block. These
functions all return NULL if they are called from outside a CATCH block.
- ERROR_NUMBER. The number of the error that
occurred. This is similar to @@ERROR except that it will return
the same number for the duration of the CATCH block.
- ERROR_MESSAGE. The complete text of the error
message including any substiture parameters such as object names.
- ERROR_LINE. This is the line number of the
batch or stored procedure where the error occured.
- ERROR_SEVERITY. This is the severity of the
error. The CATCH block only fires for errors with severity 11 or
higher. Error severities from 11 to 16 are typically user or code
errors. Severity levels from 17 to 25 are usually software or hardware
errors where processing may not be able to continue.
- ERROR_STATE. This is sometimes used by the
system to return more information about the error.
- ERROR_PROCEDURE. If the error was generated
inside a stored procedure this will hold the name of the procedure.
Trapping Errors in Stored Procedures
A TRY CATCH block can catch errors in stored procedures called by other
stored procedures. An example is:
BEGIN TRY
EXEC ParentError
END TRY
BEGIN CATCH
SELECT Error_Line = ERROR_LINE(),
Error_Proc = ERROR_PROCEDURE()
END CATCH
Assuming that the ParentError stored procedure calls the ChildError
stored procedure which generates an error you'd see output like this:
Error_Line Error_Proc
----------- -------------
5 ChildError
Error Handling and Transactions
The TRY CATCH syntax also works well with transactions. Below
is a common pattern used inside stored procedures for transactions.
Any errors cause the transaction to roll back.
BEGIN TRY
BEGIN TRANSACTION
INSERT INTO dbo.invoice_header
(invoice_number, client_number)
VALUES (2367, 19)
INSERT INTO dbo.invoice_detail
(invoice_number, line_number, part_number)
VALUES (2367, 1, 84367)
COMMIT TRANSACTION
END TRY
BEGIN CATCH
IF @@TRANCOUNT() > 0 ROLLBACK TRANSACTION
-- And do some cool error handling
END CATCH
