PolarDB-X 中操作符的优先级由高到低，如下所示：

特殊说明

IN/NOT IN 与 = 优先级

在 MySQL 5.7.19 中执行如下 SQL：

1. mysql> select binary 'a' = 'a' in (1, 2, 3);
2. +-------------------------------+
3. | binary 'a' = 'a' in (1, 2, 3) |
4. +-------------------------------+
5. | 1 |
6. +-------------------------------+
7. 1 row in set, 1 warning (0.01 sec)
8. 
   
9. mysql> show warnings;
10. +---------+------+---------------------------------------+
11. | Level | Code | Message |
12. +---------+------+---------------------------------------+
13. | Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
14. +---------+------+---------------------------------------+
15. 1 row in set (0.00 sec)
16. 
    
17. mysql> select 1 in (1, 2, 3) = 'a';
18. +----------------------+
19. | 1 in (1, 2, 3) = 'a' |
20. +----------------------+
21. | 0 |
22. +----------------------+
23. 1 row in set, 1 warning (0.00 sec)
24. 
    
25. mysql> show warnings;
26. +---------+------+---------------------------------------+
27. | Level | Code | Message |
28. +---------+------+---------------------------------------+
29. | Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
30. +---------+------+---------------------------------------+
31. 1 row in set (0.00 sec)

可见，在MySQL中，IN或NOT IN的优先级高于=（比较运算符），在PolarDB-X中，严格按照以上优先级实现，在优先级相同的情况下，采用左结合的方式。