Oracle学习第五课

分组

关键词: group by 列名

作用: 对源数据,按照指定的列,进行分组操作.

sql语法结构: select... from ... where ... group by 字段 order by ...

--统计各个部门的平均工资?
人工操作思路:
① 确定分组的依据,对原表数据进行分组.[group by 分组]
② 对每个组进行统计操作(平均值)[组函数 统计]
​
SQL书写步骤
① 确定分组依据,进行分组操作: group by department_id
② 对分组后的每组,统计salary的平均值. avg(salary);
③ 组合SQL
select avg(salary), department_id
from employees
group by department_id;

案例2:
-- 统计各个职位的人数和职位名称信息?
SQL思路:
1. 确定分组依据,进行分组操作: group by job_id
2. 对分组后的每组数据,统计人数: count(employee_id)
3. 组合SQL
select count(employee_id),job_id
from employees
group by job_id;

总结: sql执行顺序

select... from ... where ... group by 字段  order by ...
​
执行顺序:
from 确定表
where 对数据进行条件过滤操作. [符合where条件的数据]
group by 对满足where条件之后的数据进行分组操作. [数据变成组数据]
select 选择要展示的信息
order by 排序

group by 语法规则

1. group by 在 where 之后执行的
2. select 后可以写分组 group by 所依据的字段(列)
3. select 组函数统计字段.

分组过滤

关键词: having 条件

作用: 对分组之后的组数据进行过滤.

语法结构: select ... from .. where .. group by ... having 组数据判断条件 order by ...

-- 统计平均工资超过8000的部门id和平均工资?
错误:
select department_id,avg(salary)
from employees
where avg(salary)>8000
group by department_id; 
​
正确:
​
思路:
1. 分组  group by department_id
2. 分组后的组数据进行过滤 having avg(salary)>8000
3. 组函数统计展示 avg(salary)
4. 组合SQL
select avg(salary),department_id
from employees
group by department_id
having avg(salary)>8000;

重点说明:

-- 统计80,90,100 部门的总工资,部门id?
思路1:
1. 分组 group by department_id
2. 分组后的数据过滤: having department_id in (80,90,100)
3. 对满足having的组,组函数统计 sum(salary)
​
select sum(salary),department_id
from employees
group by department_id
having department_id in (80,90,100);
​
思路2:
1. 使用where关键字,对原表数据进行过滤操作 where department_id in (80,90,100)
2. 根据部门id进行分组: group by department_id
3. 对每个组统计总工资, 部门id
​
select sum(salary),department_id
from employees
where department_id in (80,90,100)
group by department_id;
​
结论:
1. where 是在分组之前对数据进行过滤操作
2. having 是在分组之后,对组进行过滤操作
3. 如果where和having同时都能满足查询需求,优先使用where.

SQL执行顺序总结

sql语法规则:select ... from .. where .. group by ... having 组数据判断条件 order by ...

sql执行顺序

from 确定表
where 对数据进行过滤 (分组之前)
group by 分组
having 分组之后的数据进行过滤
select 选择要展示信息(组函数统计,分组依据字段,xxx)
order by 排序