命令行验证MySQL的隔离级别

https://www.bilibili.com/video/BV1eJ411c7rf?p=40

准备

Windows 下 cmd 进入命令行：

C:\Users\Administrator>mysql -uroot -p123456

创建一个新用户登录MySQL Server：

mysql> create user tom identified by 'abc123';
Query OK, 0 rows affected (0.03 sec)

用户名：tom 密码：abc123

登录tom：

C:\Users\Administrator>mysql -utom -pabc123

查看 root 用户可以操作的数据库：

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db2019             |
| guns               |
| jdbc_test          |
| ke                 |
| m3u8_download      |
| mysql              |
| nacos_config       |
| performance_schema |
| sakila             |
| seata              |
| seata_account      |
| seata_order        |
| seata_storage      |
| sql                |
| sys                |
| world              |
+--------------------+
17 rows in set (0.00 sec)

查看 tom 用户可以操作的数据库：

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
+--------------------+
1 row in set (0.00 sec)

为 tom 用户授权：

mysql> grant select,insert,update,delete on jdbc_test.* to tom@localhost identified by 'abc123';
Query OK, 0 rows affected, 1 warning (0.00 sec)

验证 tom 用户得到的授权：

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| jdbc_test          |
+--------------------+
2 rows in set (0.00 sec)

验证事务隔离级别为 REPEATABLE-READ 时的情况

查看初始数据

mysql> select * from user_table;
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    1000 |
| BB   | 654321   |    1000 |
+------+----------+---------+
2 rows in set (0.00 sec)

设置事务不自动提交

tom 和 root 都设置

mysql> set autocommit =false;
Query OK, 0 rows affected (0.00 sec)

查看 tom 用户当前的事务隔离级别

mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set, 1 warning (0.00 sec)

root 用户查看当前数据

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    1000 |
+------+----------+---------+
1 row in set (0.00 sec)

tom 用户查看当前数据

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    1000 |
+------+----------+---------+
1 row in set (0.00 sec)

tom 用户修改数据

mysql> update user_table set balance = 3000 where user = 'AA';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

tom 用户查看修改后的数据

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    3000 |
+------+----------+---------+
1 row in set (0.00 sec)

root 用户查看数据

mysql>  select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    1000 |
+------+----------+---------+
1 row in set (0.00 sec)

tom 用户提交事务

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

root 用户再次查看数据

mysql>  select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    1000 |
+------+----------+---------+
1 row in set (0.00 sec)

balance 的值仍然是 1000，因为默认事务的隔离级别是 REPEATABLE-READ（可重复读），

所以用户 root 在用户 tom 更新 user='AA' 的用户 balance 为 3000 之后且提交事务之前读取的 balance 值为 1000；（屏蔽了脏读的情况，不会读取到用户 tom 未提交事务变更的数据）

所以用户 root 在用户 tom 更新 user='AA' 的用户 balance 为 3000 且提交事务之后，在用户 root 自己的当前事务中读取到的 balance 值仍为 1000；（屏蔽了不可重复读的情况，在用户 root 自己的当前事务 commit 或 rollback 之前可以重复读取同一条数据，得到的值始终是一致的）。

root 用户提交事务后查看数据

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

mysql>  select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    3000 |
+------+----------+---------+
1 row in set (0.00 sec)

现在看到了 tom 用户提交更新的数据。

验证事务隔离级别为 READ-COMMITTED 时的情况

root 用户修改全局事务隔离级别

mysql> set global transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set, 1 warning (0.00 sec)

注意：现在修改完全局的事务隔离级别之后查询事务隔离级别仍然显示为“REPEATABLE-READ”，这是因为需要把当前连接断开重连之后才会生效。

mysql> exit
Bye

C:\Users\Administrator>mysql -uroot -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 114
Server version: 5.7.35-log MySQL Community Server (GPL)

Copyright (c) 2000, 2021, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.00 sec)

其他连接也是，也需要断开连接重连后，才能应用新的全局事务隔离级别

mysql> exit
Bye

C:\Users\Administrator>mysql -utom -pabc123
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 115
Server version: 5.7.35-log MySQL Community Server (GPL)

Copyright (c) 2000, 2021, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.00 sec)

重新设置 root 和 tom 禁用自动提交

因为断开重连 MySQL Server，所以需要再次设置禁用自动提交

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> set autocommit=false;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set, 1 warning (0.00 sec)

在 tom 的事务中更新数据

mysql> update user_table set balance = 4000 where user = 'AA';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    4000 |
+------+----------+---------+
1 row in set (0.00 sec)

在 root 的事务中查询数据

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    3000 |
+------+----------+---------+
1 row in set (0.00 sec)

提交 tom 的事务

mysql> commit;
Query OK, 0 rows affected (0.01 sec)

再在 root 的事务中查询数据

mysql> select * from user_table where user = 'AA';
+------+----------+---------+
| user | password | balance |
+------+----------+---------+
| AA   | 123456   |    4000 |
+------+----------+---------+
1 row in set (0.00 sec)

因为当前 root 的事务隔离级别是 READ-COMMITTED，

所以在 root 的事务 commit 或 rollback 之前读取不到 tom 的事务中更新了、但未执行事务提交的数据；（屏蔽了脏读）

但在 tom 的事务提交之后，在 root 的事务 commit 或 rollback 之前，在 root 的事务中就读取到了 tom 的事务中更新且进行事务提交了的数据。（未屏蔽不可重复读的问题，在 root 的同一个事务中两次读取到的 user='AA' 的 balance 值不一致）