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In this Try This exercise, you will explore transaction support statements in your RDBMS
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1 Use the same Department table that was used in Try This 10-1 If you have already created
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one, drop it and re-create it so your query results in this exercise will be predictable Run the following statements (the DROP statement is unnecessary if the table does not exist):
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DROP TABLE DEPARTMENT; CREATE TABLE DEPARTMENT (DEPARTMENT_CODE CHAR(3), DEPARTMENT_NAME VARCHAR(50));
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Databases: A Beginner s Guide
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2 Set the database in implicit transaction mode For Oracle, this is the default, provided
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you have not set autocommit mode on Consult your RDBMS documentation for how this is done If you are using SQL Server, use the following statement:
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SET IMPLICIT_TRANSACTIONS ON
3 Insert one row into the table using the following statement, but do not commit
the change:
INSERT INTO DEPARTMENT VALUES ('001','Executive');
4 Run a SELECT statement to confirm that the row exists:
SELECT * FROM DEPARTMENT;
5 If you know how to connect to the database a second time in a different client session,
do so and run the select query from Step 4 in it You should not be able to find the row because it is uncommitted data, and it is therefore available only in the session that created it Depending on how your DBMS handles locking, it may appear as if this query is stalled while the SQL client waits for the DBMS to return the row (particularly in SQL Server) Locking is covered in the next section of this chapter
6 Run a ROLLBACK statement as follows:
ROLLBACK;
7 Run the SELECT from Step 4 again Notice that the row is now gone 8 Run the INSERT from Step 3 again, followed by a commit:
INSERT INTO DEPARTMENT VALUES ('001','Executive'); COMMIT;
9 Run the SELECT from Step 4 again to confirm that the row is there 10 Run a ROLLBACK as you did in Step 6 In SQL Server, you may get an error that
tells you that no transaction is in progress (the previous commit ended your implicit transaction)
ROLLBACK;
11 Try the SELECT one more time Notice that the row is still there A ROLLBACK has
no effect on data that has already been committed to the database
11: Deploying Databases
12 Drop the Department table to return your database (schema in Oracle) to where you
started In Oracle, DDL statements are never part of transactions, but they are in SQL Server, so you will need to run a COMMIT after the DROP statement in SQL Server:
DROP TABLE DEPARTMENT; COMMIT;
Try This Summary
In this Try This exercise, you used the implicit transaction mode along with INSERT, SELECT, COMMIT, and ROLLBACK statements to demonstrate transaction support in SQL
Locking and Transaction Deadlock
Although the simultaneous sharing of data among many database users has significant benefits, a serious drawback can cause updates to be lost Fortunately, database vendors have worked out solutions to the problem This section presents the concurrent update problem and various solutions
The Concurrent Update Problem
Figure 11-1 illustrates the concurrent update problem that occurs when multiple database sessions are allowed to concurrently update the same data Recall that a session is created every time a database user connects to the database, which includes the same user connecting to the database multiple times The concurrent update problem happens most often between two different database users who are unaware that they are making conflicting updates to the same data However, database users with multiple connections can trip themselves up if they apply updates using more than one of their database sessions
1 Retrieve customer balance ($200) 3 Update customer balance ($300) Database User B 2 Retrieve customer balance ($200) 4 Update customer balance ($100) Customer Schema
Process Customer Invoice ($100) Process Customer Payment ($100)
Database User A
Figure 11-1 The concurrent update problem
Databases: A Beginner s Guide
The scenario presented features a fictitious company that sells products and creates an invoice for each order shipped, similar to Acme Industries in the normalization examples from earlier chapters Figure 11-1 illustrates user A, a clerk in the shipping department who is preparing an invoice for a customer, which requires updating the customer s data by adding to the customer s balance due At the same time, user B, a clerk in the accounts receivable department, is processing a payment from the very same customer, which requires updating the customer s balance due by subtracting the amount the customer paid Here is the exact sequence of events, as illustrated in Figure 11-1:
1 User A queries the database and retrieves the customer s balance due, which is $200 2 A few seconds later, user B queries the database and retrieves the same customer s
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