三、基础查询——MySQL数据库之知识点拾遗(7)

Myth丶恋晨 2023-06-22 03:22 13阅读 0赞

## 三、基础查询——MySQL数据库之知识点拾遗(7) ##

### 1.空值运算符 ###

**(1)常见的空值运算符**

is null 和 is not null

**(2)实例**

**示例1：**

检查score列的空值并取出该数据。

select * from student_score where score is null;

![2019121317162793.png][]

**示例2：**

检查score列的非空值并取出该数据。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70][]

### 2.and和or的优先级问题 ###

选择num等于25或者35并且价格大于5的水果的所有信息。

select * from fruit where num=25 or num=35 and price>5;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 1][]

select * from fruit where (num=25 or num=35) and price>5;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 2][]

**总结：and的优先级高于or。**

[2019121317162793.png]: https://img-blog.csdnimg.cn/2019121317162793.png
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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20191213172229540.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw==,size_16,color_FFFFFF,t_70