要设置事务隔离级别，请使用 ISOLATION LEVEL level 子句。在相同的 ISOLATION LEVEL 语句中不允许指定多个 SET TRANSACTION 子句。To set the transaction isolation level, use an ISOLATION LEVEL level clause. It is not permitted to specify multiple ISOLATION LEVEL clauses in the same SET TRANSACTION statement.

默认隔离级别是 REPEATABLE READ。其他允许的值包括 READ COMMITTED、READ UNCOMMITTED 和 SERIALIZABLE。有关这些隔离级别的信息，请参阅 第 17.7.2.1 节，“事务隔离级别”。The default isolation level is REPEATABLE READ. Other permitted values are READ COMMITTED, READ UNCOMMITTED, and SERIALIZABLE. For information about these isolation levels, see Section 17.7.2.1, “Transaction Isolation Levels”.

要设置事务访问模式，请使用 READ WRITE 或 READ ONLY 子句。在相同的 SET TRANSACTION 语句中不允许指定多个访问模式子句。To set the transaction access mode, use a READ WRITE or READ ONLY clause. It is not permitted to specify multiple access-mode clauses in the same SET TRANSACTION statement.

默认情况下，事务以读写模式进行，允许对事务中使用的表进行读写操作。可以使用 SET TRANSACTION 并带有 READ WRITE 访问模式显式指定此模式。By default, a transaction takes place in read/write mode, with both reads and writes permitted to tables used in the transaction. This mode may be specified explicitly using SET TRANSACTION with an access mode of READ WRITE.

如果事务访问模式设置为 READ ONLY，则禁止对表进行更改。这可能使存储引擎在写操作不允许时进行性能优化。If the transaction access mode is set to READ ONLY, changes to tables are prohibited. This may enable storage engines to make performance improvements that are possible when writes are not permitted.

在只读模式下，仍可使用 DML 语句更改使用TEMPORARY。不允许对 DDL 语句进行更改，与永久表相同。In read-only mode, it remains possible to change tables created with the TEMPORARY keyword using DML statements. Changes made with DDL statements are not permitted, just as with permanent tables.

READ WRITE 和 READ ONLY 访问模式也可以使用 START TRANSACTION 语句为单个事务指定。The READ WRITE and READ ONLY access modes also may be specified for an individual transaction using the START TRANSACTION statement.

您可以全局设置事务特性、针对当前会话设置，或仅针对下一事务进行设置：You can set transaction characteristics globally, for the current session, or for the next transaction only:

更改全局事务特性需要 CONNECTION_ADMIN 权限（或已弃用的 SUPER 权限）。任何会话都可以更改其会话特性（即使在事务中途）或其下一事务的特性（在该事务开始之前）。A change to global transaction characteristics requires the CONNECTION_ADMIN privilege (or the deprecated SUPER privilege). Any session is free to change its session characteristics (even in the middle of a transaction), or the characteristics for its next transaction (prior to the start of that transaction).

要在服务器启动时设置全局隔离级别，请在命令行或选项文件中使用 --transaction-isolation=level 选项。此选项中 level 的值使用连字符而不是空格，因此允许的值为 READ-UNCOMMITTED、READ-COMMITTED、REPEATABLE-READ 或 SERIALIZABLE。To set the global isolation level at server startup, use the --transaction-isolation=level option on the command line or in an option file. Values of level for this option use dashes rather than spaces, so the permissible values are READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE.

类似地，要在服务器启动时设置全局事务访问模式，请使用 --transaction-read-only 选项。默认值为 OFF（读/写模式），但可以将其设置为 ON 以启用只读模式。Similarly, to set the global transaction access mode at server startup, use the --transaction-read-only option. The default is OFF (read/write mode) but the value can be set to ON for a mode of read only.

例如，要将隔离级别设置为 REPEATABLE READ 并将访问模式设置为 READ WRITE，可在选项文件的 [mysqld] 部分中使用以下行：For example, to set the isolation level to REPEATABLE READ and the access mode to READ WRITE, use these lines in the [mysqld] section of an option file:

[mysqld]
transaction-isolation = REPEATABLE-READ
transaction-read-only = OFF

在运行时，全局、会话和下一事务范围级别的特性可以如前所述通过 SET TRANSACTION 语句间接设置。也可以通过 SET 语句直接为 transaction_isolation 和 transaction_read_only 系统变量赋值：At runtime, characteristics at the global, session, and next-transaction scope levels can be set indirectly using the SET TRANSACTION statement, as described previously. They can also be set directly using the SET statement to assign values to the transaction_isolation and transaction_read_only system variables:

下表显示了每个SET TRANSACTION和变量赋值语法设置的特征作用域级别。The following tables show the characteristic scope level set by each SET TRANSACTION and variable-assignment syntax.

可以在运行时检查事务特性的全局和会话值：It is possible to check the global and session values of transaction characteristics at runtime:

SELECT @@GLOBAL.transaction_isolation, @@GLOBAL.transaction_read_only;
SELECT @@SESSION.transaction_isolation, @@SESSION.transaction_read_only;