SELECT數據查詢5——group by分組查詢

語法格式：

       select 分組函數，列(要求出現在group by的後面)
       from 表
       【where】 篩選條件
       group by 分組列表
       【order by 子句】

注意：
查詢列表必須特殊，要求是分組函數和group by 後出現的字段

總結特點：

    1、分組查詢中的篩選條件分爲兩類
      				 數據庫        	   位置                	關鍵字
    	分組前篩前    原始表            group by子句前面    	where
   		分組篩選後    分組後的結果集 	   group by子句後面   	having
   		 
    (1)分組函數做條件一定是放在having子句中
    (2)能用分組前篩選的，就優先考慮使用分組前篩選
    2、group by子句支持單個子句字段分組，多個字段分組（逗號），
    表達式或函數
    3、可以添加排序，放在最後

下面例題使用到的數據：
employees 員工表

salary	job_id	commission_pct	departemt_id	last_name	manager_id
工資列	工資列	獎金率列			部門編號			員工名列		領導編號列

departments 部門表

department_id	department_name	manager_id	location_id
部門標號列		部門名稱列		領導編號列	位置編號列

#引入：查詢每個部門的平均工資

SELECT AVG(salary) FROM employees;

#簡單分組查詢題

#1、查詢每個工種的最高工資
SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;

#2、查詢每個位置上的部門個數
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

#添加簡單(分組前)的篩選條件

#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
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

#2、查詢有獎金的每個領導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;

#添加複雜(分組後)的篩選條件

1、查詢哪個部門的員工個數>2

#(1)查詢每個員工的個數
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id;

#(2)根據(1)的結果進行篩選，查詢哪個部門的員工數>2  having
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2
ORDER BY COUNT(*);

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(1)結果繼續篩選，最高工資>12000
SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

3、查詢領導編號>120的每個領導手下的最低工資>5000的領導編號是哪個，以及其最低工資

#(1)查詢每個領導
SELECT MIN(salary),manager_id
FROM employees
GROUP BY manager_id;

#(2)添加篩選條件：領導編號>120
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id;

#(3)再添加篩選條件：最低工資>5000
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id
HAVING MIN(salary)>5000
ORDER BY MIN(salary);

#按表達式或函數分組

按員工姓名的長度分組，查詢每一組的員工個數，篩選員工個數>5的有哪些

#(1)查詢每個長度的員工個數
SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name);

#(2)添加篩選條件
SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name)
HAVING COUNT(*)>5;
#使用別名
SELECT COUNT(*) AS a,LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTH(last_name)
HAVING a>5;

#按多個字段分組

#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 job_id,department_id;

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