# 查询中的运算符

## 1、算术运算符

```
+、-、*、/、%

基本算术运算：通常不在条件中使用，而是用于结果运算（select 字段中）
```

## 2、比较运算符

```
>、>=、<、<=、=、<>
通常是用来在条件中进行限定结果
=：在mysql中，没有对应的 ==比较符号，就是使用=来进行相等判断
<=>：相等比较
```

## 3、逻辑运算符

```
and、or、not
and：逻辑与
or：逻辑或
not：逻辑非
```

## 4、In运算符

In：在什么里面，是用来替代=，当结果不是一个值，而是一个结果集的时候

基本语法： in (结果1,结果2,结果3…)，只要当前条件在结果集中出现过，那么就成立

```
mysql> select * from class;
+----------+------------+--------------+
| class_id | class_name | class_number |
+----------+------------+--------------+
|        1 | 一年级     |           60 |
|        2 | 二年级     |           40 |
|        3 | 三年级     |           50 |
|        4 | 四年级     |           45 |
|        5 | 五年级     |           49 |
|        6 | 六年级     |           56 |
+----------+------------+--------------+
6 rows in set (0.04 sec)

mysql> select * from class where class_id in ('1','3','5');
+----------+------------+--------------+
| class_id | class_name | class_number |
+----------+------------+--------------+
|        1 | 一年级     |           60 |
|        3 | 三年级     |           50 |
|        5 | 五年级     |           49 |
+----------+------------+--------------+
3 rows in set (0.06 sec)
```

## 5、Is运算符

Is是专门用来判断字段是否为NULL的运算符

基本语法：is null / is not null

```
mysql> select * from class where class_number is null;
Empty set (0.09 sec)

mysql> select * from class where class_number is not null;
+----------+------------+--------------+
| class_id | class_name | class_number |
+----------+------------+--------------+
|        1 | 一年级     |           60 |
|        2 | 二年级     |           40 |
|        3 | 三年级     |           50 |
|        4 | 四年级     |           45 |
|        5 | 五年级     |           49 |
|        6 | 六年级     |           56 |
+----------+------------+--------------+
6 rows in set (0.06 sec)
```

## 6、Like运算符

Like运算符：是用来进行模糊匹配（匹配字符串）

基本语法：like ‘匹配模式’;

匹配模式中，有两种占位符：

\_：匹配对应的单个字符

%：匹配多个字符

```
mysql> select * from class where class_name like '二%';
+----------+------------+--------------+
| class_id | class_name | class_number |
+----------+------------+--------------+
|        2 | 二年级     |           40 |
+----------+------------+--------------+
1 row in set (0.01 sec)

mysql> select * from class where class_name like '二__';
+----------+------------+--------------+
| class_id | class_name | class_number |
+----------+------------+--------------+
|        2 | 二年级     |           40 |
+----------+------------+--------------+
1 row in set (0.03 sec)

mysql> select * from class where class_name like '二_';
Empty set (0.07 sec)
```


---

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