Beginning SQL

contention. You could wrap your new tables in a transaction, but that would only tie up valuable locks
for a process that really doesn’t need locking at all. Therefore, you do not wrap the statements that create
the tables in transaction statements. Once the tables are added to the database, you need to add initial
data to the Movie Format table and the Inventory table. Each of these processes entails writing SQL
statements, and they depend on the previous operation succeeding or the next operation is not valid. For
example, you need to build the Movie Format table and place two records in that table. The three formats
are DVD, VHS, and Beta. If the statement that builds the table fails, obviously the process of placing
the three records in the format table also fails. Again, transactions are not necessary, and you receive
errors if the previous CREATE TABLE statements fail.
Next, you need to build the Inventory table and then place several inventory records in the table, which
tells you what movies are available to be rented and in what format. Then you can start renting movies.
The process of renting movies consists of running a query to find out what is available, selecting a specific
inventory item and marking it as rented, and building a new record in the Rentals table with the
member ID, the inventory ID, and the date rented.
Try It Out Renting Movies with Transactions
Select all available inventory records so that you can pick a movie. To do this, you really should wrap
the stuff that follows in a transaction. You are filling an order and don’t want the items you are modifying
to also be modified by other users.
1. Remember that in ANSI SQL you have to specifically end the previous transaction in order to
begin another transaction, so the first thing you do is run a COMMIT statement:
COMMIT
In Transact-SQL (SQL Server for example) this would not be required since you never explicitly
started a transaction.
In addition, for Transact-SQL you would need to explicitly start a transaction:
BEGIN TRANSACTION
2. For non-Transact-SQL you do not need to run a BEGIN TRANSACTION statement because it is
implicit.
SELECT Inventory.InventoryId, Films.FilmName, Films.PlotSummary, Format.Format
FROM (Inventory
INNER JOIN Films ON Inventory.FilmId = Films.FilmId)
INNER JOIN Format ON Inventory.FormatId = Format.FormatId
WHERE (((Inventory.CheckedOut)= ‘n’));
3. Select a specific movie and mark it as rented:
UPDATE Inventory
SET Inventory.CheckedOut = ‘Y’
WHERE (((Inventory.InventoryId)=7));
4. Build a record in the Rentals table:
INSERT INTO Rentals ( InventoryId, MemberId, DateOut )
SELECT 7 AS InventoryId, 3 AS MemberId, ‘3/21/2004’ AS dateout;
Transactions
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