我们下边的例子中读取数据的时候对数据加Read locks for Updating锁.

SESSION 1：

mysql> delete from t where id=2;
Query OK, 2 rows affected (0.09 sec)

mysql> select * from t;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

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

mysql> select max(id) from t for update;
+---------+
| max(id) |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

mysql> insert into t values(2);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t;
+------+
| id   |
+------+
|    1 |
|    2 |
+------+
2 rows in set (0.00 sec)

SESSION 2：

mysql> select max(id) from t for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

SESSION 1：

mysql> commit;
Query OK, 0 rows affected (0.09 sec)

SESSION 2：

mysql> select max(id) from t for update;
+---------+
| max(id) |
+---------+
|       2 |
+---------+
1 row in set (0.02 sec)

mysql> insert into t values(3);
Query OK, 1 row affected (0.09 sec)

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

mysql> select * from t;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
+------+
3 rows in set (0.00 sec)