Mysql-DQL (grouping queries)

Advanced 5: grouping queries

Syntax:
the SELECT list of queries
from table
[where] filter criteria
field group by grouping
[order by sorting fields];
Note: The
list of queries have special requirements field after the group functions and group by

Features:
1, the packet filter criteria in the query into two categories

	data source	position	Keyword
Packet filtering ago	Original table	group by front	where
After the screening group	After grouping result set	group by back	having

Grouping function to make sure the conditions placed in the HAVING
Group by clause supports a single field grouping, grouping multiple fields, functions, expressions, can take on the final sorting

Grouping function:
COUNT
SUM
max
min
AVG

#查询每个部门的员工个数
SELECT COUNT(*) FROM employees WHERE department_id=90;

#案例1：查询每个工种的员工平均工资
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;

Add filter criteria grouped

#案例1 查询那个部门的员工数>2
#1.查询每个部门的员工个数
#2.根据1的结果进行筛选，查询那个部门的员工个数>2
/*
不能用where因为where是从你原始表里面筛选 并且更在from后面 而现在要求是在结果的新建表里面进行筛选 用having
*/
select count(*),department_id from employees 
group by department_id 
having count(*)>2;

#案例2: 查询每个工种有奖金的员工的最高工资>12000的工种编号和最高工资
 select
  max(salary),
  job_id
from
  employees
where commission_pct is not  null
group by job_id
having max(salary)>12000;

#按多个字段分组
#案例:查询每个部门每个工种的平均工资
select avg(salary),department_id,job_id from employees 
group by department_id,job_id; 

#添加排序
#案例:查询每个部门每个工种的平均工资,并且按平均工资的高低显示
 SELECT
  AVG(salary),
  department_id,
  job_id
FROM
  employees
WHERE department_id IS NOT NULL
GROUP BY department_id,
  job_id
HAVING AVG(salary)>10000
order by avg(salary) desc;