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The SQL transaction concept
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The ANSI/ISO SQL Transaction Model
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The ANSI/ISO SQL standard defines a SQL transaction model and seven statements that support transaction processing: START TRANSACTION transaction. Sets the properties of a new transaction and starts that
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SET TRANSACTION Sets the properties of the next transaction to be executed. It has no effect, however, on any currently executing transaction. SET CONSTRAINTS Sets the constraint mode within a current transaction. The constraint mode controls whether a constraint is applied immediately to data as it is modified or whether enforcement of the constraint is to be deferred until later in the transaction. SET CONSTRAINTS is presented in 11. SAVEPOINT Creates a savepoint within a transaction. A savepoint is a place within a transaction s sequence of events that can act as an intermediate recovery point. A current transaction can be rolled back to the savepoint instead of to the beginning of the transaction. RELEASE SAVEPOINT holding. Releases a savepoint, freeing up any resources it may be
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COMMIT Terminates a successful transaction and commits all changes to the database. ROLLBACK When used without a savepoint, terminates an unsuccessful transaction and rolls back any changes to the beginning of a transaction, essentially restoring the database to its consistent state before the transaction (as if the transaction had never executed). When used with a savepoint, rolls back the transaction to the named savepoint, but allows it to continue. The first version of the SQL standard (SQL1) defined an implicit transaction mode, based on the transaction support in the early releases of DB2. In implicit mode, only the COMMIT and ROLLBACK statements are supported. A SQL transaction automatically begins with the first SQL statement executed by a user or a program and ends when a COMMIT or ROLLBACK is executed. The end of one transaction implicitly starts a new one. In programmatic SQL, successful termination of a program is handled as if a COMMIT was issued, and abnormal termination of a program is handled as if a ROLLBACK was issued. Many commercial products, notably DB2 and Oracle, still default to implicit transaction mode when you first connect to the database. The SQL2 and SQL:1999 versions of the ANSI/ISO SQL standard introduced the other statements shown in the preceding list. Most modern commercial products support them; however, there are exceptions. For example, Sybase ASE and SQL SERVER support a BEGIN TRANSACTION statement instead of START TRANSACTION, and a SAVE TRANSACTION statement instead of SAVEPOINT.
The START TRANSACTION and SET TRANSACTION Statements
The syntax diagrams for the START TRANSACTION and SET TRANSACTION statements are shown in Figure 12-2. The fundamental difference between the two is that START TRANSACTION starts a new transaction with an option to set certain properties of the
12:
Tr a n s a c t i o n P r o c e s s i n g
START TRANSACTION ISOLATION LEVEL READ UNCOMMITTED READ COMMITTED REPEATABLE READ SERIALIZABLE [,] READ ONLY READ WRITE
[,] DIAGNOSTICS SIZE number-of-conditions
SET [LOCAL] TRANSACTION ISOLATION LEVEL READ UNCOMMITTED READ COMMITTED REPEATABLE READ SERIALIZABLE [,] READ ONLY READ WRITE
PART III
[,] DIAGNOSTICS SIZE number-of-conditions
FIGURE 12-2
The START TRANSACTION and SET TRANSACTION statements
transaction, while the SET TRANSACTION statement sets properties that will be used by the next transaction it cannot be used within an existing transaction, and therefore can have no effect on any existing transaction. One other difference is that the SET TRANSACTION statement has a LOCAL option, which can be used to set options for local server execution of a transaction that spans multiple servers. The three properties that may be set by these statements (in any order desired) are Isolation level Defines how isolated a transaction will be from the actions of other transactions. The specific options (READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE) are presented in detail in the Isolation Levels topic later in this chapter. If no isolation level is specified, the default is SERIALIZABLE. Access level Defines whether the transaction can contain statements that modify the database (READ WRITE) or not (READ ONLY). The default depends on the isolation level selected, but if no isolation level is specified, the default is READ WRITE. Diagnostics size Defines the size of the diagnostics area used for conditions that can be raised by SQL statements as they execute. A condition is a warning, exception, or other type of message generated by SQL statement execution. For example, if diagnostics size is set to 10, then up to 10 conditions can be stored for an executed statement. Note that as of this writing, the diagnostics size option is not supported by SQL Server, Sybase ASE, Oracle, MySQL, DB2, or most other current SQL implementations.
Part III:
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