分組查詢

1 語法

select 查詢列表 from 表 【where 篩選條件】 group by 分組的欄位【order by 排序的欄位】;

2 特點

• 1、和分組函式一同查詢的欄位必須是group by後出現的欄位
• 2、篩選分為兩類：分組前篩選和分組後篩選

• 問題

  • 問題1：分組函式做篩選能不能放在where後面

    答：不能

  • 問題2：where——group by——having

    一般來講，能用分組前篩選的，儘量使用分組前篩選，提高效率

• 3、分組可以按單個欄位也可以按多個欄位

• 4、可以搭配著排序使用

3 案例

#1.簡單的分組

#案例1：查詢每個工種的員工平均工資
SELECT AVG(salary),job_id FROM employees GROUP BY job_id;
 

#案例2：查詢每個位置的部門個數
SELECT COUNT(*),location_id FROM departments GROUP BY location_id;

#2、可以實現分組前的篩選

#案例1：查詢郵箱中包含a字元的 每個部門的最高工資
SELECT MAX(salary),department_id FROM employees WHERE email LIKE '%a%' GROUP BY department_id;
#案例2：查詢有獎金的每個領導手下員工的平均工資
SELECT AVG(salary),manager_id FROM employees WHERE commission_pct IS
 
 NOT NULL GROUP BY manager_id;

#3、分組後篩選

#案例1：查詢哪個部門的員工個數>5
#①查詢每個部門的員工個數
SELECT COUNT(*),department_id FROM employees GROUP BY department_id;
#② 篩選剛才①結果
SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*)>5;

#案例2：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
SELECT job_id,MAX(salary) FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING MAX(salary)>12000;

#案例3：領導編號>102的每個領導手下的最低工資大於5000的領導編號和最低工資
SELECT manager_id,MIN(salary) FROM employees WHERE manager_id>102 GROUP BY manager_id HAVING MIN(salary)>5000;

#4.新增排序

#案例：每個工種有獎金的員工的最高工資>6000的工種編號和最高工資,按最高工資升序
SELECT job_id,MAX(salary) m FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING m>6000 ORDER BY m ;

#5.按多個欄位分組

#案例：查詢每個工種每個部門的最低工資,並按最低工資降序
SELECT MIN(salary),job_id,department_id FROM employees GROUP BY department_id,job_id ORDER BY MIN(salary) DESC;