数据库查询优化之子查询优化

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;

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;