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SQL: A Beginner s Guide
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during the transaction, and identify places within a transaction that act as stopping points when you roll back transactions. Throughout the rest of this chapter, we ll examine how each of these statements is used within a transaction. However, before we go into a more detailed discussion of the statements, I want to provide you with a brief overview of each one in order to give you a better understanding of how transactions work. The SQL:2006 standard defines seven statements related to transaction processing:
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SET TRANSACTION
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Sets the properties of the next transaction to be executed. Sets the properties of a transaction and starts that transaction.
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START TRANSACTION
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SET CONSTRAINTS Sets the constraint mode within a current transaction. The constraint mode refers to whether a constraint is applied immediately to data when that data is modified or whether the application of the constraint is deferred until later in the transaction. SAVEPOINT Creates a savepoint within a transaction. A savepoint marks a place within the transaction that acts as a stopping point when you roll back a transaction. RELEASE SAVEPOINT Releases a savepoint.
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ROLLBACK Terminates a transaction and rolls back any changes to the beginning of the transaction or to a savepoint. COMMIT Terminates a transaction and commits all changes to the database.
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Although we ll be looking at all seven statements in more detail, some of them are pivotal in understanding the nature of a transaction. Let s take a look at Figure 16-1 to help illustrate this point.
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SET TRANSACTION START TRANSACTION Database at beginning of transaction
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SQL statements
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Successful statement execution
Unsuccessful statement execution
ROLLBACK
COMMIT
Database updated
Figure 16-1
A basic SQL transaction
16:
Managing SQL Transactions
Notice that the figure includes four of the SQL transaction-related statements: SET TRANSACTION, START TRANSACTION, COMMIT, and ROLLBACK. If a SET TRANSACTION statement is used, it is executed before the transaction begins. After that, a START TRANSACTION statement begins the transaction.
NOTE
As you ll see later in this chapter, it would be rare in a pure SQL environment that you would want to use both the SET TRANSACTION and START TRANSACTION statements because both statements set the same properties. However, you ll find that SQL implementations vary with regard to which transaction-related statements they support and how they implement those statements. Specific differences are pointed out throughout this chapter.
When you start the transaction, the database is in its original state the data is consistent and correct. Next the SQL statements within the transaction are processed. If this process is successful, a COMMIT statement is executed. The COMMIT statement causes the SQL implementation to update the database and terminate the transaction. If the statement-execution process is not successful, a ROLLBACK statement is executed, and the implementation returns the database to its original state. An unsuccessful execution does not necessarily mean that the statements have failed. A ROLLBACK statement can be executed according to the conditions of a WHERE clause, a predefined error, or whatever other conditions are defined within the transaction. The point is, under certain circumstances, the ROLLBACK statement is executed and under other circumstances, the COMMIT statement is executed.
Set Transaction Properties
The first statement that we ll look at in detail is the SET TRANSACTION statement. The SET TRANSACTION statement allows you to configure several of the properties associated with transaction processing. You can execute this statement only when no transactions are active (or in the case of Oracle, only as the first statement after a COMMIT or ROLLBACK). When you do use a SET TRANSACTION statement, the settings configured within the statement are applied only to the next transaction that is initiated. The settings do not carry over from one transaction to the next. The SET TRANSACTION statement is not required in order to initiate a transaction. If the statement is not executed, the transaction uses either the default settings or the settings supplied in the subsequent START TRANSACTION statement. If the SET TRANSACTION statement is executed, the transaction uses the settings specified in that statement. If the statement is executed but not all settings are defined, the transaction uses the defaults for the undefined settings. Regardless of which settings are configured, none of them are applicable to any transaction except the first one initiated after the SET TRANSACTION is executed. Now let s take a look at the syntax used for a SET TRANSACTION statement. At its most basic, the syntax looks like the following: SET [ LOCAL ] TRANSACTION <mode> [ { , <mode> } . . . ]
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