MySQL（七）DQL之分组查询
一、语法
select 分组函数，分组后的字段from 表【where 筛选条件】group by 分组的字段【having 分组后的筛选】【order by 排序列表】
二、特点
分组前筛选：where – 原始表 – group by的前面
分组后筛选：having – 分组后的结果 – group by 的后面
三、简单的分组
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;
案例2：查询每个位置的部门个数
SELECT COUNT(*),location_id FROM departments GROUP BY location_id;
四、分组前筛选
案例1：查询邮箱中包含a字符的 每个部门的最高工资
SELECT MAX(salary),department_id FROM employees WHERE email LIKE ‘%a%’ GROUP BY department_id;
案例2：查询有奖金的每个领导手下员工的平均工资
SELECT AVG(salary),manager_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY manager_id;
五、分组后筛选
案例1：查询哪个部门的员工个数>5
SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*)>5;
案例2：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 job_id,MAX(salary) FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING MAX(salary)>12000;
案例3：领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资
SELECT manager_id,MIN(salary) FROM employees WHERE manager_id > 102 GROUP BY manager_id HAVING MIN(salary)>5000;
六、添加排序
案例：每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序
SELECT job_id, MAX(salary) m FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING m >6000 ORDER BY m;
七、按多个字段分组
案例：查询每个工种每个部门的最低工资,并按最低工资降序
SELECT MIN(salary),job_id,department_id FROM employees GROUP BY department_id,job_id ORDER BY MIN(salary) DESC;
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