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Try It Out: What Happens When Both Operations Fail
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In this example, you ll try to insert an invalid new customer and try to delete an undeletable one. 1. Run sp_Trans_Test to add customer a and delete customer ALFKI. The result should appear as in Figure 14-5.
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CHAPTER 14 s USING TRANSACTIONS
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Figure 14-5. Both operations rolled back
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2. In the Messages window, note that the transaction was rolled back (even though neither statement succeeded, so there was nothing to roll back) because @maxerr was 2627, returned for the INSERT. Error messages for both failing statements are displayed at the top of the window.
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By now, you should understand why both statements failed. This example proves that even when the first statement fails the second is executed (and in this case failed with error number 547), which our original example, where the error code was zero when there were no rows to delete, didn t necessarily prove since the error number there may have come from the
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set @maxerr = @inserr
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immediately before the DELETE statement. We ve used rather straightforward but crude T_SQL to demonstrate transactions, primarily because we wanted to focus on the underlying operational dynamics, but we don t mean to suggest this is necessarily the best way to code transactions in T-SQL. Let s see a more Visual Basic like alternative.
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Try It Out: Coding a Transaction with the TRY CATCH Construct
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Let s code a transaction to both add a customer to and delete one from the Northwind Customers table. Customers has eleven columns, but only two, CustomerID and CompanyName, don t allow nulls, so we ll use just those columns for insertion. We ll also use arbitrary customer IDs to make it easy to find the rows we manipulate when viewing customers sorted by ID.
CHAPTER 14 s USING TRANSACTIONS
1. In SSMSE, create a stored procedure named sp_Trans_Try, using the code in Listing 14-2. Note that you re using several new SQL statements for stored procedure programming. We don t explain the ones not involved in transactions, but their meaning and usage should be obvious.
Listing 14-2. sp_Trans_Try
create procedure sp_Trans_Try @newcustid nchar(5), @newconame nvarchar(40), @oldcustid nchar(5) as declare @inserr int declare @delerr int declare @maxerr int set @maxerr = 0 begin try begin transaction -- Add a customer insert into customers ( customerid, companyname ) values(@newcustid, @newconame) -- Save error number set @inserr = @@error if @inserr > @maxerr set @maxerr = @inserr -- Delete a customer delete from customers where customerid = @oldcustid
CHAPTER 14 s USING TRANSACTIONS
-- Save error number set @delerr = @@error if @delerr > @maxerr set @maxerr = @delerr commit print 'Transaction committed' end try begin catch rollback print 'Transaction rolled back' end catch print 'INSERT error number:' + cast(@inserr as nvarchar(8)) print 'DELETE error number:' + cast(@delerr as nvarchar(8)) return @maxerr
2. Run the stored procedure from Object Explorer. When prompted, enter aaa for both @newcustid and @newconame and aa for @oldcustid. The Messages window (see Figure 14-6) is almost the same as Figure 14-1. The difference is that here we ve actually deleted a row, for customer aa, so we see 1 row(s) affected for the DELETE.
Figure 14-6. Using TRY CATCH with a transaction
CHAPTER 14 s USING TRANSACTIONS
3. Look at the table with Object Explorer and you ll see that customer aaa has been inserted and customer aa deleted. (See Figure 14-7.)
Figure 14-7. Rows inserted and deleted using TRY CATCH
How It Works
Most of the code remained the same. We left in the instrumentation, but we really don t need @maxerr, at least not to decide whether to roll back or commit, so the code is lengthier than it has to be. The biggest changes were moving the statements, including COMMIT, into a TRY block:
begin try begin transaction -- Add a customer insert into customers ( customerid, companyname ) values(@newcustid, @newconame) -- Save error number set @inserr = @@error if @inserr > @maxerr set @maxerr = @inserr -- Delete a customer delete from customers where customerid = @oldcustid -- Save error number set @delerr = @@error if @delerr > @maxerr set @maxerr = @delerr
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