MySQL（七）DQL之分組查詢

一、語法

select 分組函數，分組後的字段from 表【where 篩選條件】group by 分組的字段【having 分組後的篩選】【order by 排序列表】

二、特點

分組前篩選：where -   原始表 - group by的前面

分組後篩選：having -  分組後的結果 - group by 的後面

三、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;

四、分組前篩選

案例1：查詢郵箱中包含a字符的 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;

五、分組後篩選

案例1：查詢哪個部門的員工個數>5

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;

六、添加排序

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;

七、按多個字段分組

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;

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