vb.net qr code sample s USING TRANSACTIONS in Visual Basic .NET

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CHAPTER 14 s USING TRANSACTIONS
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@@ERROR is reset after every T-SQL statement (even SET and IF) is executed, so if you
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want to save an error number for a particular statement you must store it before the next statement executes. That s why you declared the local variables. You used @maxerr to simplify the test, but you could have tested the others instead. If @@ERROR returns any value other than 0, an error has occurred, and you want to roll back the transaction. You also include PRINT statements to report that a rollback or commit has occurred.
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-- If an error occurred, roll back if @maxerr <> 0 begin rollback print 'Transaction rolled back' end else begin commit print 'Transaction committed' end
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s T-SQL (and standard SQL) supports various alternative forms for keywords and phrases. We ve used Tip
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just ROLLBACK and COMMIT here.
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Then you added some more instrumentation, so you could see what error numbers were encountered during the transaction:
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print 'INSERT error number:' + cast(@inserr as nvarchar(8)) print 'DELETE error number:' + cast(@delerr as nvarchar(8)) return @maxerr
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Now let s look at what happened when you executed the stored procedure. You ran it twice, and added a new customer each time, but you also entered the same nonexistent customer to delete each time. If all statements in a transaction are supposed to succeed or fail as one unit, why did the INSERT succeed when the DELETE didn t delete anything Figure 14-2 should make everything clear. Both the INSERT and DELETE had zero error numbers. (When a DELETE doesn t find any rows to delete, T-SQL doesn t treat that as an error.) In fact, that s why you used a nonexistent customer. The rest of the Customers
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CHAPTER 14 s USING TRANSACTIONS
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(well, all but two) have child Orders, and you can t delete customers before you delete their orders. Now that you have a couple customers we can delete at will, let s explore some of the essential features of transactions. There s much more to transactions than we can cover in this chapter, but the following examples will give you a foundation to extrapolate from to handle any combination of operations.
Try It Out: What Happens When the First Operation Fails
In this example, you ll try to insert an invalid (duplicate) customer and delete a deletable one. 1. Run sp_Trans_Test to add customer a and delete customer aa. The result should appear as in Figure 14-3.
Figure 14-3. Second operation rolled back
2. In the Messages window, note that the transaction was rolled back because the INSERT failed and was terminated with error number 2627 (whose error message appears at the top of the window). The DELETE error number was 0, so it apparently executed successfully, but was rolled back. (If you check the table, you ll find aa is still a customer.)
How It Works
Since customer a already exists, SQL Server prevented the insertion of a duplicate, so the first operation failed. When the second statement in the transaction was executed, customer aa was deleted since it didn t have any child orders, but since @maxerr wasn t zero (it s 2627, as the Results window shows), you rolled back the transaction, undoing the deletion.
CHAPTER 14 s USING TRANSACTIONS
Try It Out: What Happens When the Second Operation Fails
In this example, you ll insert a valid new customer and try to delete an undeletable one. 1. Run sp_Trans_Test to add customer aaa and delete customer ALFKI. The result should appear as in Figure 14-4.
Figure 14-4. First operation rolled back
2. In the Messages window, note that the transaction was rolled back because the DELETE failed and was terminated with error number 547 (whose error message appears at the top of the window). The INSERT error number was 0, so it apparently executed successfully, but was rolled back. (If you check the table, you ll find aaa is not a customer.)
How It Works
Since customer aaa doesn t exist, SQL Server inserted the row, so the first operation succeeded. When the second statement in the transaction was executed, SQL Server prevented the deletion of customer ALFKI because it had child orders, but since @maxerr wasn t zero (it s 547, as the Results window shows), you rolled back the transaction, undoing the delete.
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