/*
語法：
select 分組函數(shù),列（要求出現(xiàn)在GRPUP BY的后面）
from 表
【where 篩選條件】
group by 分組的列表
【order by 字句】
注意：
查詢列表必須特殊，要求是分組函數(shù)和group by后出現(xiàn)的字段
特點(diǎn)：
1、分組查詢中的篩選條件分為兩類

• 分組函數(shù)做條件肯定是放在having子句中
• 能用分組前篩選的，優(yōu)先考慮使用分組前篩選

2、group by 字句支持單個(gè)字段分組，多個(gè)字段分組（多個(gè)字段之間逗號隔開沒有順序要求），表達(dá)式或函數(shù)（用得較少）
3、也可以添加排序（排序放在整個(gè)分組查詢的最后）

引入，查詢每個(gè)部門的平均工資

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

簡單的分組查詢

案例1：查詢每個(gè)工種的最高工資

 SELECT MAX(salary),job_id
 FROM employees
 GROUP BY job_id;

案例2：每個(gè)位置上的部門個(gè)數(shù)

SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

添加篩選條件

案例1:查詢郵箱中包含a字符的，每個(gè)部門的平均工資

SELECT AVG(salary),department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

案例2:查詢有獎金的每個(gè)領(lǐng)導(dǎo)手下員工的最高工資

SELECT manager_id,MAX(salary)
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;

添加復(fù)雜的篩選條件

案例1：查詢哪個(gè)部門的員工個(gè)數(shù)>2

• 先創(chuàng)建結(jié)果集

SELECT department_id,COUNT(*) AS 員工個(gè)數(shù)
FROM employees
GROUP BY department_id;

• 根據(jù)結(jié)果集進(jìn)行篩選，

SELECT department_id,COUNT(*) 
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 2;

案例2：查詢每個(gè)工種有獎金的員工的最高工資>12000的工種編號和最高工資

SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

變種：查詢每個(gè)工種有獎金的員工的最高工資>12000的 email

SELECT email
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

案例3：查詢領(lǐng)導(dǎo)編號>102的每個(gè)領(lǐng)導(dǎo)手下的最低工資>5000的領(lǐng)導(dǎo)編號是哪個(gè)，以及其最低工資

SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id
HAVING MIN(salary)>5000;

按表達(dá)式進(jìn)行分組

案例：按員工姓名的長度分組，查詢每一組的員工個(gè)數(shù)，篩選員工數(shù)>5的有哪些

SELECT COUNT(*) c,LENGTH(last_name) len
FROM employees
GROUP BY len
HAVING c > 5;#只有mysql支持在group by 和 having后面放別名

按多個(gè)字段分組

案例：查詢每個(gè)部門每個(gè)工種的員工的平均工資

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

添加排序

案例：查詢每個(gè)部門每個(gè)工種的員工的平均工資，并且按平均工資的高低顯示出來

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