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Managing SQL Transactions
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In SQL:2006, a transaction can be started either implicitly or explicitly. A transaction starts implicitly when certain types of SQL statements are executed, such as the SELECT, DELETE, UPDATE, and CREATE TABLE statements. These types of statements must be executed within the context of a transaction. If a transaction is not active, one is initiated. Transactions can also be initiated explicitly by using the START TRANSACTION statement. The START TRANSACTION statement serves two purposes: to set the transaction s properties, and to initiate the transaction. In terms of setting the properties, the START TRANSACTION statement works just like the SET TRANSACTION statement. You can set the access level, the isolation level, and the diagnostics size. As for initiating a transaction, you simply execute the START TRANSACTION statement. The syntax for the START TRANSACTION statement is similar to the SET TRANSACTION statement, as you can see in the following syntax: START TRANSACTION <mode> [ { , <mode> } . . . ] After you specify the START TRANSACTION keywords, you must specify one or more transaction modes. As with the SET TRANSACTION statement, you can include only one mode for each type. Now let s take a look at an example that defines an access level of READ ONLY, an isolation level of READ UNCOMMITTED, and a diagnostics size of 5:
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START TRANSACTION READ ONLY, ISOLATION LEVEL READ UNCOMMITTED, DIAGNOSTICS SIZE 5;
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As you can see, this looks almost identical to a SET TRANSACTION statement. The transaction modes are applied in the same way, and if more than one transaction mode is specified, they re separated with commas. The basic difference between a START TRANSACTION statement and a SET TRANSACTION statement is that the START TRANSACTION statement will initiate the transaction as well as set its properties.
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The START TRANSACTION statement was added to SQL with the release of SQL:1999. As a result, not all SQL implementations support a START TRANSACTION statement or any statement that explicitly initiates a transaction. Transactions in Oracle, for example, can only be initiated implicitly. On the other hand, SQL Server supports a BEGIN TRANSACTION statement, but it does not allow you to define any transaction modes.
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Does it matter whether your transaction includes data definition language (DDL) statements or data manipulation language statements SQL allows you to include both types of statements in your transaction, but this is not the case for all SQL implementations. Some implementations do not allow you to mix the two types of statements in a single transaction. Other products allow you to mix the two types of statements, but limit which statements can be combined in one transaction. And still other implementations do not allow data definition language statements to be executed within the context of a transaction. For example, Oracle implicitly ends any current transaction when it encounters a DDL statement, and never handles DDL as part of a transaction. The restrictions that various implementations place on mixing statement types can vary widely. The reason for this is that the interactions between the two types of statements can be complicated, so each implementation determines what statement mixtures it will support in its own database environment. Be sure to check the product documentation to determine what types of statements can be included in a transaction and how they can be mixed.
Set Constraint Deferability
There can be times in a transaction when you want to modify data in a table that temporarily violates a constraint placed on that table. For example, you might have a table that includes a column configured with the NOT NULL constraint. It is possible that, during the course of the transaction, you want to insert a row in the table but don t yet have a value for the NOT NULL column. For this reason, the SQL standard allows you to define a constraint as deferrable. That means that the constraint does not have to be applied to the data immediately (when the modifying SQL statement is executed); it can be postponed until a later point in a transaction, such as after you were able to insert a value into the NOT NULL column. If a constraint is defined as deferrable, you can use the SET CONSTRAINTS statement within the transaction to defer the application of the constraint or to apply the constraint immediately. (Defining a constraint as deferrable doesn t automatically defer the application of that constraint. You must still explicitly defer it within the transaction.) If you explicitly defer a constraint, you can then temporarily violate the deferred constraint until the constraint is explicitly applied or the transaction ends. For a better understanding of how this works, let s take a look at Figure 16-5. In this illustration, you ll notice that, after the transaction has been started, you can set the constraints to deferred. You don t have to defer the constraints right after the transaction starts, but you must defer them before executing any SQL statements that could violate the constraints. Once the applicable SQL statements have been executed and you re sure that no SQL data violates any of the deferred constraints, you can then apply the constraints to the applicable data. If the constraints are violated at this point, the transaction is considered unsuccessful and any updates are rolled back. Otherwise, the updates are committed to the database.
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