SET [GLOBAL|SESSION] <variable>

    SET [GLOBAL|SESSION] 语句用于在 SESSION 或 GLOBAL 的范围内，对某个 TiDB 的内置变量进行更改。

    VariableAssignment:

    VariableAssignment

    1. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    2. | Variable_name | Value                                                                                                                                     |
    3. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    4. | sql_mode      | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
    5. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    6. 1 row in set (0.00 sec)
    1. SHOW SESSION VARIABLES LIKE 'sql_mode';
    1. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    2. | Variable_name | Value                                                                                                                                     |
    3. | sql_mode      | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
    4. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    5. 1 row in set (0.00 sec)

    更新全局的 sql_mode：

    1. Query OK, 0 rows affected (0.03 sec)

    检查更新之后的 sql_mode 的取值，可以看到 SESSION 级别的值没有更新：

    1. SHOW GLOBAL VARIABLES LIKE 'sql_mode';
    1. +---------------+-----------------------------------------+
    2. | Variable_name | Value                                   |
    3. +---------------+-----------------------------------------+
    4. | sql_mode      | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER |
    5. +---------------+-----------------------------------------+
    6. 1 row in set (0.00 sec)
    1. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    2. | Variable_name | Value                                                                                                                                     |
    3. | sql_mode      | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
    4. +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+
    5. 1 row in set (0.00 sec)

    SET SESSION 则可以立即生效：

    1. SET SESSION sql_mode = 'STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER';
    1. Query OK, 0 rows affected (0.01 sec)
    1. +---------------+-----------------------------------------+
    2. | Variable_name | Value                                   |
    3. +---------------+-----------------------------------------+
    4. | sql_mode      | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER |
    5. +---------------+-----------------------------------------+
    6. 1 row in set (0.00 sec)
    • 与 MySQL 不同，TiDB 中使用 SET GLOBAL 所作的修改会应用于集群中的全部 TiDB 实例。而在 MySQL 中，修改不会应用于副本。
    • TiDB 中的若干变量可读又可设置，这是与 MySQL 相兼容的要求，因为应用程序和连接器常读取 MySQL 变量。例如：JDBC 连接器同时读取和设置缓存查询的参数，尽管并不依赖这一行为。