【15】通配符注意事项

1、AND&OR操作符

筛选出sex=m且品种为猫或狗的记录：

mysql> select * from pet;
+----------+-------------+---------+------+------------+------------+
| name     | owner       | species | sex  | birth      | death      |
+----------+-------------+---------+------+------------+------------+
| Puffball | Diane       | hamster | f    | 2000-09-22 | NULL       |
| Cuihua   | ZhouXingChi | DOG     | M    | 1999-09-21 | NULL       |
| DNN      | Huimin      | Cat     | f    | 2018-07-07 | NULL       |
| Claws    | Gwen        | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold      | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny       | dog     | m    | 1909-08-27 | NULL       |
| Bowser   | Diane       | dog     | m    | 1979-08-31 | 1995-07-29 |
| Chirpy   | Gwen        | bird    | f    | 1989-09-01 | NULL       |
| Slim     | Benny       | snake   | m    | 2006-04-21 | NULL       |
| Puffball | Diane       | hamster | f    | 2009-10-12 | NULL       |
+----------+-------------+---------+------+------------+------------+
10 rows in set (0.00 sec)

mysql> select * from pet where species='cat' or species='dog' and sex='m';
+--------+-------------+---------+------+------------+------------+
| name   | owner       | species | sex  | birth      | death      |
+--------+-------------+---------+------+------------+------------+
| Cuihua | ZhouXingChi | DOG     | M    | 1999-09-21 | NULL       |
| DNN    | Huimin      | Cat     | f    | 2018-07-07 | NULL       |
| Claws  | Gwen        | cat     | m    | 1994-03-17 | NULL       |
| Fang   | Benny       | dog     | m    | 1909-08-27 | NULL       |
| Bowser | Diane       | dog     | m    | 1979-08-31 | 1995-07-29 |
+--------+-------------+---------+------+------------+------------+
5 rows in set (0.00 sec)

mysql> select * from pet where (species='cat' or species='dog') and sex='m';
+--------+-------------+---------+------+------------+------------+
| name   | owner       | species | sex  | birth      | death      |
+--------+-------------+---------+------+------------+------------+
| Cuihua | ZhouXingChi | DOG     | M    | 1999-09-21 | NULL       |
| Claws  | Gwen        | cat     | m    | 1994-03-17 | NULL       |
| Fang   | Benny       | dog     | m    | 1909-08-27 | NULL       |
| Bowser | Diane       | dog     | m    | 1979-08-31 | 1995-07-29 |
+--------+-------------+---------+------+------------+------------+

WHERE可包含任意数目的AND和OR操作符，允许两者结合以进行复杂和高级的过滤。

AND操作符的优先级大于OR操作符，在使用具有AND和OR操作符的WHERE子句都因该使用圆括号明确的分组。

使用IN操作符，与OR相同的功能：

mysql> select * from pet where species in ('dog','cat') and sex='m';
+--------+-------------+---------+------+------------+------------+
| name   | owner       | species | sex  | birth      | death      |
+--------+-------------+---------+------+------------+------------+
| Cuihua | ZhouXingChi | DOG     | M    | 1999-09-21 | NULL       |
| Claws  | Gwen        | cat     | m    | 1994-03-17 | NULL       |
| Fang   | Benny       | dog     | m    | 1909-08-27 | NULL       |
| Bowser | Diane       | dog     | m    | 1979-08-31 | 1995-07-29 |
+--------+-------------+---------+------+------------+------------+

使用IN操作符的优点：

1.使用更长的合法选项清单时，更清楚直观；

2.计算的次序更容易管理（使用的操作符更少）；

3.IN操作符比OR操作符清单执行更快；

4.可以包含其他SELECT语句，更动态的建立WHERE子句。

NOT与IN连用：

mysql> select * from pet where species not in ('dog','cat') and sex='m';
+------+-------+---------+------+------------+-------+
| name | owner | species | sex  | birth      | death |
+------+-------+---------+------+------------+-------+
| Slim | Benny | snake   | m    | 2006-04-21 | NULL  |
+------+-------+---------+------+------------+-------+

2、通配符

%可匹配0个、1个、多个字符，_只能匹配1个字符。

使用通配符时需记住：

1.不要过度使用通配符。如果其他操作符能达到相同的目的，应该使用其他操作符；

2.在确实需要使用通配符时，除非绝对有必要，否则绝不要把它们用在搜索模式的开始处。置于开始处，搜索起来时最慢的；

3.仔细注意通配符的位置，如果放错地方，可能不能返回想要的数据。