【mysql】查询数据时group by，及group_concat()函数用法

GROUP BY 语句

语法

        select 
            聚合函数，
            列（要求出现在group by的后面）
        from 
            表
        where 
            筛选条件
        group by 
            分组的列表
        order by 
            子句

注意：除了出现在group by后面的字段，如果要在select后查询其他字段，必须用聚合函数进行聚合

特点：分组查询中的筛选条件分为两类：
分组前筛选：数据源是原始表，用where，放在group by前面，因为在分组前筛选
分组后筛选：数据源是分组后的结果集，用having，放在group by后面，因为在分组后进行筛选。

例子

# 查询每个工种最高工资员工
select 
    max(salary),
    job_id
from 
    employees
group by 
    job_id;



# 查询邮箱中包含a字符的每个部门的平均工资
select 
    avg(salary),
    department_id
from 
    employees
where 
    email like '%a%'
group by 
    department_id



# 查询有奖金的每个领导手下最高工资的员工工资
select 
    max(salary),
    manager_id 
from 
    employees
where 
    commission_pct is not null
group by 
    manager_id



# 查询哪个部门的员工个数大于2
select  
    count(*),
    department_id 
from 
    employees 
group by 
    department_id 
having 
    count(*) > 2;



# 查询每个工种有奖金的员工的最高工资大于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;

按表达式或聚合函数进行筛选

# 按照员工姓名长度分组，查询每一组员工个数，筛选员工个数大于5的有哪些
select 
    count(*)  ,
    length(last_name) as len_name
from 
    employees 
group by 
    length(last_name)
having 
    count(*) > 5;

按照多个字段分组

# 查询每个部门每个工种的员工平均工资,并且按照平均工资的高低排序
select 
    avg(salary),
    department_id,
    job_id 
from 
    employees
where 
    department_id is not null
group by 
    department_id,
    job_id
order by 
    avg(salary) desc;