#進階5：分組查詢

/*語法：select 分組函數(shù)，列(要求出現(xiàn)在group by的后面)from 表【where 篩選條件】group by 分組的列表【order by 子句】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： 1.分組查詢中的篩選條件分為兩類數(shù)據(jù)源位置關(guān)鍵字分組前篩選原始表group by子句前面where分組后篩選分組后的結(jié)果集group by子句后面having①分組函數(shù)做條件肯定是放在having子句中②能用分組前篩選的優(yōu)先使用分組前篩選2、group by子句支持單個字段分組，多字段分組(多個字段之間用逗號隔開沒有順序要求)，表達式或者函數(shù)(用的較少)3、也可以添加排序(排序放在整個分組查詢的最后)*/

#引入：查詢每個部門的平均工資

SELECT AVG(salary) FROM employees;

#簡單的分組查詢

#案例1：查詢每個工種的最高工資

SELECT MAX(salary),job_id

FROM employees

GROUP BY job_id;

#案例2：查詢每個位置上的部門個數(shù)

SELECT COUNT(*),location_id

FROM departments

GROUP BY location_id;

#添加分組前的篩選條件

#案例1：查詢郵箱中包含a字符的，每個部門的平均工資

SELECT AVG(salary),department_id,email

FROM employees

WHERE email LIKE '%a%'

GROUP BY de partment_id

#案例2：查詢有獎金的每個領(lǐng)導(dǎo)手下員工的最高工資

SELECT MAX(salary),manager_id

FROM employees

WHERE commission_pct IS NOT NULL

GROUP BY manager_id

#添加分組后的篩選條件

#案例1：查詢那個部門的員工個數(shù)>2

#1.查詢每個部門的員工個數(shù)(涉及個數(shù)使用count(*))

SELECT COUNT(*),department_id

FROM employees

GROUP BY department_id

#2.根據(jù)1的結(jié)果進行篩選，查詢哪個部門的員工個數(shù)>2

SELECT COUNT(*),department_id

FROM employees

GROUP BY department_id

HAVING COUNT(*)>2;

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

#1.查詢每個工種有獎金的員工的最高工資

SELECT MAX(salary),job_id

FROM employees

WHERE commission_pct IS NOT NULL

GROUP BY job_id

#2.根據(jù)1的結(jié)果繼續(xù)篩選，最高工資>12000

HAVING MAX(salary) >12000;

#案例3：查詢領(lǐng)導(dǎo)編號>102的每個領(lǐng)導(dǎo)手下的最低工資大于5000的領(lǐng)導(dǎo)編號以及最低工資

#1.查詢領(lǐng)導(dǎo)編號>102的每個領(lǐng)導(dǎo)手下的員工固定最低工資

SELECT MIN(salary),manager_id

FROM employees

GROUP BY manager_id

#2.添加篩選條件：編號>102

SELECT MIN(salary),manager_id

FROM employees

WHERE manager_id >102

GROUP BY manager_id

3#添加篩選條件：最低工資>5000

SELECT MIN(salary),manager_id

FROM employees

WHERE manager_id >102

GROUP BY manager_id

HAVING MIN(salary) >5000;、

#按表達式或者函數(shù)分組

#案例1：按員工姓名長度分組，查詢每一組的員工個數(shù)，篩選員工個數(shù)>5的有哪些

#①查詢每個長度的員工個數(shù)

SELECT COUNT(*),LENGTH(last_name) len_name

FROM employees

GROUP BY LENGTH(last_name);

#②添加篩選條件

SELECT COUNT(*) c,LENGTH(last_name) len_name

FROM employees

GROUP BY len_name

HAVING c;

#按多個字段分組

#案例：查詢每個部門，每個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：查詢每個部門，每個工種的員工的平均工資并且按平均工資(大于10000)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;

#1.查詢各job_id員工工資的最大值，最小值，平均值，總和，并按job——id升序

SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id

FROM employees

GROUP BY job_id

ORDER BY job_id ASC;

#2查詢最高工資和最低工資的差距(difference)

SELECT MAX(salary) - MIN(salary) difference

FROM employees;

#查詢各個管理者手下員工的最低工資，其中最低工資不能第于6000，沒有管理者的員工不計算在內(nèi)

SELECT MIN(salary),manager_id

FROM employees

WHERE manager_id IS NOT NULL

GROUP BY manager_id

HAVING MIN(salary) >=6000

#4查詢所有部門的編號，員工數(shù)量和工資平均值，并按平均工資降序

SELECT department_id,COUNT(*),AVG(salary)

FROM employees

GROUP BY department_id

ORDER BY AVG(salary) DESC;

#5.選擇具有各個job_id的員工人數(shù)

SELECT job_id,COUNT(*)

FROM employees

GROUP BY job_id;

總結(jié)

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