mysql 分组函数和分组查询使用详解

SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT COMMIT(salary) FROM employees;
SELECT SUM(salary) 和，AVG(salary) 平均，MAX(salary) 最大，MIN(salary) 最小 FROM employees;

#1.count 函数的详细介绍
//count:
//MYISAM 存储引擎下,COUNT() 的效率高
//INNODB 存储引擎下,COUNT()和 COUNT(1)的效率差不多，比COUNT(字段)要高一些
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT COUNT(1) FROM employees;

#和分组函数一同查询的字段有限制
#案例1.查询公司员工的工资最大值，最小值，平均值，总和
SELECT MAX(salary) mx_sal,MIN(salary) mi_sal,AVG(salary) ag_sal,SUM(salary) sm_sal FROM employees;
#案列2. 查询员工表中的最大入职时间和最小入职时间的相差天数
SELECT DATEDIFF(MAX(hiredate),MIN(hiredate)) DIFFRENCE FROM employees;
#案例3 查询部门编号为90的员工个数
SELECT COUNT(*) FROM employees WHERE department_id =90;

#案列1:查询每个部门的平均工资
SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;
#案列2: 查询每个工种的最高工资
SELECT MAX(salary),job_id
FROM employees;
GROUP BY job_id;
#案列3 查询每个位置上的部门个数
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

#添筛选条件
#案例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;
#案例2 查寻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;
#案例3 查询那个部门的员工个数>2
    #1.查询每个部门的员工个数
SELECT COUNT(*),department_id
FROM employees;
GROUP BY department_id;
#2.根据1的结果进行帅选，查询那个部门的员工个数>2
SELECT COUNT(*),department_id
FROM employees;
GROUP BY department_id
HAVING COUNT(*)>2;
#案例4；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结果继续帅选，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;