SELECT数据查询5——group by分组查询

语法格式：

       select 分组函数，列(要求出现在group by的后面)
       from 表
       【where】 筛选条件
       group by 分组列表
       【order by 子句】

注意：
查询列表必须特殊，要求是分组函数和group by 后出现的字段

总结特点：

    1、分组查询中的筛选条件分为两类
      				 数据库        	   位置                	关键字
    	分组前筛前    原始表            group by子句前面    	where
   		分组筛选后    分组后的结果集 	   group by子句后面   	having
   		 
    (1)分组函数做条件一定是放在having子句中
    (2)能用分组前筛选的，就优先考虑使用分组前筛选
    2、group by子句支持单个子句字段分组，多个字段分组（逗号），
    表达式或函数
    3、可以添加排序，放在最后

下面例题使用到的数据：
employees 员工表

salary	job_id	commission_pct	departemt_id	last_name	manager_id
工资列	工资列	奖金率列			部门编号			员工名列		领导编号列

departments 部门表

department_id	department_name	manager_id	location_id
部门标号列		部门名称列		领导编号列	位置编号列

#引入：查询每个部门的平均工资

SELECT AVG(salary) FROM employees;

#简单分组查询题

#1、查询每个工种的最高工资
SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;

#2、查询每个位置上的部门个数
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

#添加简单(分组前)的筛选条件

#1、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;

#2、查询有奖金的每个领导手下员工的最高工资
SELECT MAX(salary),manager_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;

#添加复杂(分组后)的筛选条件

1、查询哪个部门的员工个数>2

#(1)查询每个员工的个数
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id;

#(2)根据(1)的结果进行筛选，查询哪个部门的员工数>2  having
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2
ORDER BY COUNT(*);

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

#(1)查询每个工种有奖金的员工的最高工资
SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id;

#(2)根据(1)结果继续筛选，最高工资>12000
SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

3、查询领导编号>120的每个领导手下的最低工资>5000的领导编号是哪个，以及其最低工资

#(1)查询每个领导
SELECT MIN(salary),manager_id
FROM employees
GROUP BY manager_id;

#(2)添加筛选条件：领导编号>120
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id;

#(3)再添加筛选条件：最低工资>5000
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id
HAVING MIN(salary)>5000
ORDER BY MIN(salary);

#按表达式或函数分组

按员工姓名的长度分组，查询每一组的员工个数，筛选员工个数>5的有哪些

#(1)查询每个长度的员工个数
SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name);

#(2)添加筛选条件
SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name)
HAVING COUNT(*)>5;
#使用别名
SELECT COUNT(*) AS a,LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name)
HAVING a>5;

#按多个字段分组

#查询每个部门每个工种的员工的平均工资
SELECT AVG(salary),department_id,job_id
FROM employees
GROUP BY job_id,department_id;

#综合
SELECT AVG(salary),department_id,job_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY job_id,department_id
HAVING AVG(salary)>10000
ORDER BY AVG(salary) DESC;