结论: 在范围判断时,尽量不要使用 not in 和 not exists,使用 left join on xxx is null 代替。
取所有不为掌门人的员工,按年龄分组!
select age as '年龄', count(*) as '人数' from t_emp where id not in (select ceo from t_dept where ceo is not null) group by age;
如何优化? 1 解决 dept 表的全表扫描,建立 ceo 字段的索引:
2 语句改成left join的方式
select age as '年龄',count(*) as '人数' from emp e left join dept d on e.id=d.ceo where d.id is null group by age; (推荐)
为方便理解上面的sql,个人做了个简单的Demo
两张表 img_type,img
--sql-1 不推荐
SELECT imgname , COUNT(*) FROM img WHERE typeid NOT IN (SELECT typeid FROM img_type WHERE typeid IS NOT NULL) GROUP BY imgname;--sql-2 推荐
SELECT imgname ,COUNT(*) FROM img LEFT JOIN img_type ON img.typeid=img_type.typeid WHERE img_type.typeid IS NULL GROUP BY imgname;--sql-3 方便理解,测试用
SELECT * FROM img LEFT JOIN img_type ON img.typeid=img_type.typeid
前两个sql运行结果
第三个sql运行结果