五、多表连接查询——数据分组

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## 五、多表连接查询——数据分组 ##

### 1.数据分组背景 ###

当汇总成绩时，需要年级之间的横向对比，比如高中一年级，然后将一年级1班、2班、...，一直到所有班级的成绩进行对比，这个时候就需要分组汇总进行统计聚合。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70][]

### 2.如何使用分组查询 ###

**(1)语法如下：**

select 列名或者聚合函数 from 表名
    [where 条件表达式]
    group by 分组列;

**(2)示例1：**

根据class列进行聚合。

select * from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 1][]

#count(*)表示计算查询的数据有多少条
    select *,count(*) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 2][]

**(3)示例2：**

使用聚合函数求每个年级的总成绩。

select *,sum(score) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 3][]

**(4)示例3：**

计算每个年级的平均分。

select *,sum(score)/count(class) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 4][]

select *,avg(score) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 5][]

select class,avg(score) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 6][]

**(5)示例4：**

汇总每个班级的人数。

select class,count(class) from grade_score group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 7][]

**(6)示例5：**

将成绩大于80的同学进行分组统计平均分数。

**【注意点：where条件过滤先发生，然后再进行分组统计】**

select class,avg(score) from grade_score where score>80 group by class;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjgzMDY5Nw_size_16_color_FFFFFF_t_70 8][]

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