All consistent reads within the same transaction read the snapshot established by the first read.

Session 1:                                                            Session 2：

mysql> show variables like '%tx_isolation%';
+---------------+-----------------+
| Variable_name | Value           |
+---------------+-----------------+
| tx_isolation  | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)

Session 1:                                                               mysql> select * from t100;  --看到的记录为291

mysql> update t100 set free = free -quota where sn=10;                    | 10 |  300 | 1     | 291  |

Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

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

mysql> select * from t100;
+----+------+-------+------+
| sn | id   | quota | free |
+----+------+-------+------+
|  1 |    7 | NULL  | NULL |
|  2 |    9 | NULL  | NULL |
|  3 |  990 | NULL  | NULL |
|  4 |   12 | NULL  | NULL |
|  5 |   13 | NULL  | NULL |
|  6 |   14 | NULL  | NULL |
|  7 |   15 | NULL  | NULL |
|  8 |   22 | NULL  | NULL |
|  9 |   23 | NULL  | NULL |
| 10 |  300 | 1     | 291  |
+----+------+-------+------+
10 rows in set (0.00 sec)

mysql> update t100 set free = free -quota where sn=10;              Session 2看到的仍旧是291
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

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

mysql> select * from t100;
+----+------+-------+------+
| sn | id   | quota | free |
+----+------+-------+------+
|  1 |    7 | NULL  | NULL |
|  2 |    9 | NULL  | NULL |
|  3 |  990 | NULL  | NULL |
|  4 |   12 | NULL  | NULL |
|  5 |   13 | NULL  | NULL |
|  6 |   14 | NULL  | NULL |
|  7 |   15 | NULL  | NULL |
|  8 |   22 | NULL  | NULL |
|  9 |   23 | NULL  | NULL |
| 10 |  300 | 1     | 290  |
+----+------+-------+------+

RR模式下 以第一次读为准

All consistent reads within the same transaction read the snapshot established by the first read.

所有的一致性读 在相同事务读取快照 有第一次读创建