Beginning SQL

Summary
Transactions ensure that users who need to modify data don’t see data that is about to be modified or in
the process of being modified. Transactions lock data sets so that you can perform modifications to
records without overwriting other users’ changes or allowing other users to overwrite your changes.
In this chapter, you learned the following:
Transactions
❑ That in order to be considered a transaction, the set of statements must pass the ACID test:
Atomic, Consistent, Isolated, and Durable.
❑ That the original ANSI standard used only COMMIT and ROLLBACK statements, but the various
database vendors have extended the standards, in particular adding the BEGIN TRANSACTION
and SAVE TRANSACTION statements to add flexibility to the process of starting a transaction and
allowing partial rollbacks.
❑ How transaction logs allow the DBMS to record the transaction and how the statements in the
transaction affect the data to protect against catastrophic failures that could corrupt the database.
❑ How to use locks to control access to different levels of objects in the database, starting at the
database level and working down to the individual record in a specific table. You learned that
locks have an associated significant overhead, and that hundreds or thousands of users trying to
get locks can cause database slowdown and general efficiency issues.
❑ How locking levels can allow you to reduce the impact of locks by allowing users to see or not
see locked records only when appropriate.
❑ How to prevent or minimize the four common problems with concurrent data access through a
good understanding of the issues involved, through an understanding of how transactions and
locks work, and by the careful use of isolation levels.
This has been a very technical chapter, one that is difficult to demonstrate in the typical tutorial method
because of that fact that transactions and their implementation are so very vendor- and product-specific.
At the same time, transactions are a key concept to correctly handling concurrency in a multiple-user
database. Failure to understand the issues can lead to huge consistency problems.
The next chapter covers SQL security, how users are created, and once created, how users are restricted
from using or are allowed to use objects of the database. Security allows you to determine who gets to
see what, as well as who is allowed to modify data in the tables that make up the database.
Exercises
1. Rewrite the update to the Inventory and insert data into the Rentals table using a READ
COMMITTED isolation level.
2. Write a SELECT statement to view all titles in the Films table using the READ UNCOMMITTED isolation
level.
3. Write an UPDATE query to update all inventory to CheckedOut (= ‘Y’) using the
SERIALIZABLE isolation level.
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