运算符优先级
本文介绍了PolarDB-X中运算符的优先级。
PolarDB-X操作符的优先级由高到低，如下所示：
| 优先级 | 运算符 |
|---|---|
| 15 | ! |
| 14 | -(负号), ~ |
| 13 | ^ |
| 12 | *，/，%，MOD |
| 11 | +，- |
| 10 | <<,>> |
| 9 | & |
| 8 | | |
| 7 | =(比较运算符等于)，<=>,>，>=，<，<=，<>，!=，IS，LIKE，REGEXP，IN |
| 6 | BETWEEN |
| 5 | NOT |
| 4 | AND, \&\& |
| 3 | XOR |
| 2 | OR, || |
| 1 | =（赋值运算符） |
特殊说明
IN/NOT IN与=优先级
在MySQL 5.7.19中执行如下SQL：
mysql> select binary 'a' = 'a' in (1, 2, 3);
+-------------------------------+
| binary 'a' = 'a' in (1, 2, 3) |
+-------------------------------+
| 1 |
+-------------------------------+
1 row in set, 1 warning (0.01 sec)
mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
+---------+------+---------------------------------------+
1 row in set (0.00 sec)
mysql> select 1 in (1, 2, 3) = 'a';
+----------------------+
| 1 in (1, 2, 3) = 'a' |
+----------------------+
| 0 |
+----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
+---------+------+---------------------------------------+
1 row in set (0.00 sec)
可见，在MySQL中，IN/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。