sql try catch rollback commit - preventing erroneous commit after rollback

Question

I am trying to write an MS sql script that has a transaction and a try catch block   If it catches an exception  the transaction is rolled back   If not  the transaction is committed   I have seen a few different websites saying to do it like this   begin transaction begin try     --main content of script here end try begin catch     rollback transaction end catch  commit transaction   But won t we still hit the  commit transaction  line even in the case of catching an exception   Won t this lead to a SQL error because the transaction has already been rolled back   I think it should be done like this   declare  success bit   1  begin transaction begin try     --main content of script here end try begin catch     rollback transaction     set  success   0 end catch  if  success   1  begin     commit transaction end   Howcome the commonly-posted solution does not include the  success variable   Is there no sql error that happens as a result of committing a transaction that has already been rolled back   Am I incorrect in saying that the  commit transaction  line of the first code example will still be hit in the case of catching an exception

User · Answer

I used below ms sql script pattern several times successfully which uses Try-Catch,Commit Transaction- Rollback Transaction,Error Tracking.

Your TRY block will be as follows

 BEGIN TRY
 BEGIN TRANSACTION T
 ----
 //your script block
 ----
 COMMIT TRANSACTION T 
 END TRY

Your CATCH block will be as follows

BEGIN CATCH
DECLARE @ErrMsg NVarChar(4000), 
        @ErrNum Int, 
        @ErrSeverity Int, 
        @ErrState Int, 
        @ErrLine Int, 
        @ErrProc NVarChar(200)
 SELECT @ErrNum = Error_Number(), 
       @ErrSeverity = Error_Severity(), 
       @ErrState = Error_State(), 
       @ErrLine = Error_Line(), 
       @ErrProc = IsNull(Error_Procedure(), '-')
 SET @ErrMsg = N'ErrLine: ' + rtrim(@ErrLine) + ', proc: ' + RTRIM(@ErrProc) + ', 
       Message: '+ Error_Message()

Your ROLLBACK script will be part of CATCH block as follows

IF (@@TRANCOUNT) > 0 
BEGIN
PRINT 'ROLLBACK: ' + SUBSTRING(@ErrMsg,1,4000)
ROLLBACK TRANSACTION T
END
ELSE
BEGIN
PRINT SUBSTRING(@ErrMsg,1,4000);   
END

END CATCH

Above different script blocks you need to use as one block. If any error happens in the TRY block it will go the the CATCH block. There it is setting various details about the error number,error severity,error line ..etc. At last all these details will get append to @ErrMsg parameter. Then it will check for the count of transaction (@@TRANCOUNT >0) , ie if anything is there in the transaction for rollback. If it is there then show the error message and ROLLBACK TRANSACTION. Otherwise simply print the error message.

We have kept our COMMIT TRANSACTION T script towards the last line of TRY block in order to make sure that it should commit the transaction(final change in the database) only after all the code in the TRY block has run successfully.

User · Answer

Below might be useful   Source  https   msdn microsoft com en-us library ms175976 aspx  BEGIN TRANSACTION   BEGIN TRY     -- your code -- 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       IF   TRANCOUNT  gt  0         ROLLBACK TRANSACTION  END CATCH   IF   TRANCOUNT  gt  0     COMMIT TRANSACTION  GO

User · Answer

Transaction counter  --  TRANCOUNT   0 begin try --  TRANCOUNT   0 BEGIN TRANSACTION tran1  --  TRANCOUNT   1          --your code         -- if failed    TRANCOUNT   1         -- if success   TRANCOUNT   0  COMMIT TRANSACTION tran1  end try  begin catch     print  FAILED  end catch

User · Answer

I always thought this was one of the better articles on the subject  It includes the following example that I think makes it clear and includes the frequently overlooked   trancount which is needed for reliable nested transactions  PRINT  BEFORE TRY  BEGIN TRY     BEGIN TRAN      PRINT  First Statement in the TRY block       INSERT INTO dbo Account AccountId  Name   Balance  VALUES 1   Account1    10000       UPDATE dbo Account SET Balance   Balance   CAST  TEN THOUSAND  AS MONEY  WHERE AccountId   1      INSERT INTO dbo Account AccountId  Name   Balance  VALUES 2   Account2    20000       PRINT  Last Statement in the TRY block      COMMIT TRAN END TRY BEGIN CATCH     PRINT  In CATCH Block      IF   TRANCOUNT  gt  0          ROLLBACK TRAN       THROW  -- raise error to the client END CATCH PRINT  After END CATCH  SELECT   FROM dbo Account WITH NOLOCK  GO

User · Answer

In your first example  you are correct   The batch will hit the commit transaction  regardless of whether the try block fires   In your second example  I agree with other commenters   Using the success flag is unnecessary     I consider the following approach to be  essentially  a light weight best practice approach   If you want to see how it handles an exception  change the value on the second insert from 255 to 256   CREATE TABLE  TEMP   ID TINYINT NOT NULL    INSERT  INTO  TEMP  ID   VALUES    1    BEGIN TRY     BEGIN TRANSACTION      INSERT  INTO  TEMP  ID   VALUES    2       INSERT  INTO  TEMP  ID   VALUES    255        COMMIT TRANSACTION END TRY BEGIN CATCH     DECLARE           ErrorMessage NVARCHAR 4000            ErrorSeverity INT           ErrorState INT      SELECT           ErrorMessage   ERROR MESSAGE             ErrorSeverity   ERROR SEVERITY             ErrorState   ERROR STATE        RAISERROR            ErrorMessage           ErrorSeverity           ErrorState                    ROLLBACK TRANSACTION END CATCH  SET NOCOUNT ON  SELECT ID FROM  TEMP  DROP TABLE  TEMP

[sql] sql try/catch rollback/commit - preventing erroneous commit after rollback

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