-- group by 子句
-- 要注意！group by 子句一定是放在WHERE的后面
/*
SELECT 分组函数,列(要求出现在GROUP BY后面)
FROM 表
[WHERE 筛选条件]
GROUP BY 分组列表
[ORDER BY 子句]

查询列表必须特殊，要求是分组函数和GROUP BY后出现的
*/

-- 查询每个部门的平均工资：
SELECT AVG(`salary`) 
FROM `employees`; 

SELECT AVG(`salary`),`department_id`
FROM `employees`
GROUP BY `department_id`;

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

# 前置加入筛选条件
-- 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`;

# 查询有奖金的每个人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`;

# 复杂筛选条件
# 案例 查询哪些部门的员工个数大于2
SELECT COUNT(*) as ‘counts‘,`department_id`
FROM `employees`
group by `department_id`
Having counts > 2; -- 对分组之后再进行筛选的，使用Having子句充当where

# 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`) ‘maxSalary‘,`job_id`
FROM `employees`
where `commission_pct` IS NOT NULL
GROUP BY `job_id`
having maxSalary > 12000 ;

# 查询每个领导编号>102的每个领导手下的最低工资>5000的领导编号是哪个，以及最低工资
SELECT MIN(`salary`),`manager_id`
FROM `employees`
WHERE `manager_id` > 102
GROUP by `manager_id`
having MIN(`salary`) > 5000;

-- 按员工姓名的长度分组，查询每一组员工个数，筛选员工个数大于5的

-- 先查询每个长度的员工个数
SELECT COUNT(‘员工个数‘),LENGTH(`last_name`) AS "名字长度分组"
FROM `employees`
GROUP BY LENGTH(`last_name`);

-- 再筛选大于5的个数
SELECT COUNT(‘员工个数‘) AS ‘c‘,LENGTH(`last_name`) AS "名字长度分组"
FROM `employees`
GROUP BY LENGTH(`last_name`)
HAVING c > 5;

-- 查询每个部门的每个工种的员工的平均工资

-- 先确定要查询的要素
SELECT AVG(`salary`),`job_id`,`department_id`
-- 查询的要素来自于哪张表
FROM `employees`
-- 对查询的条件进行判断
GROUP BY `job_id`,`department_id`;

-- 查询每个部门的每个工种的员工的平均工资,按平均工资从高到底显示
SELECT AVG(`salary`),`job_id`,`department_id`
FROM `employees`
GROUP BY `job_id`,`department_id`
ORDER BY AVG(`salary`) DESC;

-- 增加部门ID不是null的情况
SELECT AVG(`salary`),`job_id`,`department_id`
FROM `employees`
WHERE `department_id` IS NOT NULL
GROUP BY `job_id`,`department_id`
ORDER BY AVG(`salary`) DESC;

-- 平均工资还要高于10000的
SELECT AVG(`salary`),`job_id`,`department_id`
FROM `employees`
WHERE `department_id` IS NOT NULL
GROUP BY `job_id`,`department_id`
HAVING AVG(`salary`) > 10000 -- 要放在排序的前面，排序留在最后面执行
ORDER BY AVG(`salary`) DESC;

-- 1、查询个工种ID的员工工资的聚合函数，并按照工种升序
SELECT `job_id`,MAX(`salary`),MIN(`salary`),AVG(`salary`),SUM(`salary`)
FROM `employees`
GROUP BY `job_id`
ORDER BY `job_id` ASC;

-- 2查询 员工工资最高最低的差距
SELECT MAX(`salary`),MIN(`salary`),MAX(`salary`) - MIN(`salary`) ‘相差金额‘
FROM `employees`;

-- 3查询各个管理者手下员工的最低工资，不得低于6000，没有管理者不计算在内

-- 歧义SQL 不正确的筛选>6000
SELECT `manager_id`,MIN(`salary`)
FROM `employees`
WHERE `salary` > 6000 AND `manager_id` IS NOT NULL -- 在分组之前筛选6000
GROUP BY `manager_id`;

-- 正确SQL
SELECT `manager_id`,MIN(`salary`)
FROM `employees`
WHERE `manager_id` IS NOT NULL -- 在分组之前筛选6000无效
GROUP BY `manager_id`
HAVING MIN(`salary`) > 6000;

-- 4 查询所有部门编号，员工数量，和工资平均值，按工资平均值降序
SELECT AVG(`salary`),COUNT(‘员工数量‘),`department_id`
FROM `employees`
GROUP BY `department_id`
ORDER BY AVG(`salary`) ASC;

-- 如果没有部门就不算在内的话...
SELECT AVG(`salary`),COUNT(‘员工数量‘),`department_id`
FROM `employees`
WHERE `department_id`  IS NOT NULL
GROUP BY `department_id` 
ORDER BY AVG(`salary`) ASC;

-- 5 查询每个具有工种ID的员工人数
SELECT `job_id`,COUNT(‘人数‘)
FROM `employees`
GROUP BY `job_id`;