Do I need to use a try..catch block, and explicit rollback in a SQL Server procedure?

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旧巷少年郎
旧巷少年郎 2021-02-05 10:04

If I am coding a SQL Server (2008r2) procedure, and I wrap it in a transaction, do I need to explicitly enclose it in a try..catch block, and then explicitly call rollback in th

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  • 2021-02-05 10:33

    It depends on the severity level of the error. Sufficiently high -- 16, perhaps? -- the process may stop at the failing line, leaving the transaction open and your locks in place. If there's any chance of error within a transaction, you definitely want to wrap it in the try-catch block, as you did in your first example.

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  • 2021-02-05 10:36

    rollbacks will occur automatically if there is an error IN MOST CASES BUT NOT ALL

    if you want to guarantee a rollback for all errors precede the begin transaction with SET XACT_ABORT ON

    Best practice is to explicity catch errors with a try-catch block and take action there, including perhaps a rollback and reporting/logging the error.

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  • 2021-02-05 10:38

    The answer to your question depends on the SET XACT_ABORT setting:

    Specifies whether SQL Server automatically rolls back the current transaction when a Transact-SQL statement raises a run-time error.

    When SET XACT_ABORT is ON, if a Transact-SQL statement raises a run-time error, the entire transaction is terminated and rolled back.

    When SET XACT_ABORT is OFF, in some cases only the Transact-SQL statement that raised the error is rolled back and the transaction continues processing. Depending upon the severity of the error, the entire transaction may be rolled back even when SET XACT_ABORT is OFF. OFF is the default setting.

    Compile errors, such as syntax errors, are not affected by SET XACT_ABORT.

    For example, try the following code. The first division by 0 raises an error but continues execution. The second division by zero raises an error and halts execution:

    begin transaction
    
    set xact_abort off
        
    select 1 / 0 -- causes divide by zero error, but continues
    select @@trancount -- returns 1
    
    set xact_abort on
    
    select 1 / 0 -- causes divide by zero error and terminates execution
    select @@trancount -- we never get here
    
    rollback
    

    If XACT_ABORT is ON, then errors will abort the transaction, and you don't need a TRY / CATCH.

    If XACT_ABORT is OFF, you will need to check the status of each statement to see if an error occurred:

    begin transaction
    
    delete from...
    if @@error <> 0
    begin
        if @@trancount > 0
            rollback
        return
    end
    
    insert into...
    if @@error <> 0
    begin
        if @@trancount > 0
            rollback
        return
    end
    
    commit
    

    However, if you ever find a case where you need to TRY / CATCH, you may need to do something special when the error occurs. If so, don't forget to TRY / CATCH the exception handling:

    begin transaction
    
    set xact_abort on
    
    begin try
        select 1 / 0 -- causes divide by zero error and terminates execution
        select @@trancount -- we never get here
        commit
    end try
    begin catch
        select xact_state() -- this will be -1 indicating you MUST rollback before doing any other operations
        select @@trancount -- this will probably be one, because we haven't ended the transaction yet
        if xact_state() <> 0
        begin try
            select 'rollback'
            rollback
            
            -- do something to handle or record the error before leaving the current scope
            select 'exception processing here'
            --insert into...
        end try
        begin catch
            -- ignore rollback errors
        end catch
        
    end catch
    
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