Question

I’m trying understand how try … catch construction works in T-SQL.

So I’ve read article at MSDN: http://msdn.microsoft.com/en-us/library/ms175976.aspx

I’m a bit confusing with this statement:
A TRY…CATCH construct cannot span multiple batches. A TRY…CATCH construct cannot span multiple blocks of Transact-SQL statements. For example, a TRY…CATCH construct cannot span two BEGIN…END blocks of Transact-SQL statements and cannot span an IF…ELSE construct.

It’s saying that try … catch can’t span 2 BEGIN…END blocks and cannot span an IF…ELSE construct.

However, I’ve tried it and it works! (on both 2005 and 2008)

Can someone explain why? It looks like some mistake at the MSDN.

Look at the my test script:

print 'start'
begin transaction test_tran
begin try
    -- first begin ... end
    begin
        -- some statements
        print 'begin ... end - #1'
    end
    -- second begin ... end
    begin
        print 'begin ... end - #2'
        -- statement with error
        select 1/0 -- division by zero
    end
    print 'end of try'
end try
begin catch
    print 'catch'
    goto RollbackTran
end catch

-- commit
print 'commit'
commit transaction test_tran

-- rollback
RollbackTran:
    BEGIN
        print 'rollback'
        WHILE @@TRANCOUNT > 0 
        begin
            ROLLBACK TRANSACTION test_tran
            print 'actual rollback'
        end
    END

Results:

start
begin ... end - #1
begin ... end - #2

(0 row(s) affected)
catch
rollback
actual rollback

It works (2 begin…end blocks)! But MSDN is saying that it should not.
Same thing for IF … ELSE statement.

Can someone explain it?

Answer 1

A T-SQL batch is most typically denoted by a GO statment. They are saying that you cannot do this:

BEGIN TRY

    PRINT 'statement 1'

    GO -- Cannot have a GO to end batch inside of a TRY / CATCH

    PRINT 'statement 2'

END TRY
BEGIN CATCH

    PRINT 'Catch Block'

END CATCH

Likewise you cannot:

IF (1 = 1)
BEGIN
   BEGIN TRY

     PRINT 'test'

END -- Cannot END a block that began prior to the TRY / CATCH

   END TRY
   BEGIN CATCH

     PRINT 'Catch block'

   END CATCH

nor can you do this:

IF (1 = 1)
    BEGIN TRY

        PRINT 'statement 1'

ELSE -- Cannot do ELSE for an IF that started prior to the TRY / CATCH
        PRINT 'statement 2'

    END TRY
    BEGIN CATCH

        PRINT 'Catch Block'

    END CATCH