Beginning SQL

Chapter 11
5. At this point, you need to run a COMMIT statement to cause your modifications to be committed
or written to the database. In SQL Server (Transact-SQL) the TRANSACTION keyword is optional
and thus can be dropped.
COMMIT
How It Works
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Transactions always have a BEGIN, whether it be implicit, as in the ANSI standard, or an explicit BEGIN
TRANSACTION, as in the Transact-SQL extension. If you are not using Transact-SQL, then you need to
perform a COMMIT to explicitly end one transaction and begin the next. Using Transact-SQL, you explicitly
executed a BEGIN TRANSACTION.
Having told the DBMS that a transaction is desired, you then executed the statements that need to be
executed as a unit. View and select a record, mark it as rented, and then create a rental record. Once all of
these statements are finished, a COMMIT statement causes the entire sequence to be written to disk and
the transaction is finished.
Likewise, the statements checking the movie back in to inventory should probably be a transaction.
Again, use a simple COMMIT statement for users on ANSI SQL:
COMMIT
Users of Transact-SQL should use an explicit BEGIN TRANSACTION statement:
BEGIN TRANSACTION
When the movie is returned, the rental record has to be updated with the date returned and the amount
paid, and the inventory record must be updated to indicate that the movie is back in stock:
UPDATE Rentals
SET Rentals.DateIn = ‘3/23/2004’, Rentals.Charge = 5
WHERE (((Rentals.InventoryId)=7) AND ((Rentals.MemberId)=3));
UPDATE Inventory SET Inventory.CheckedOut = ‘n’
WHERE (((Inventory.InventoryId)=7));
At this point, you need to run a COMMIT statement to cause your modifications to be committed or written
to the database.
COMMIT
In SQL Server (Transact-SQL) the TRANSACTION keyword is optional and thus can be dropped if desired.
Using transactions in this manner prevents potential problems such as another database user trying to
rent the same movie at the same time or a movie being marked as returned in the Rentals table but never
being marked as available to be rented in the Inventory table.