mysql - 如何将数据库子查询重写为连接？

Question

我正在尝试将此子查询重写为连接。我已经阅读了关于 SO 的其他问题，但无法解决这个问题。

create table job (
  emplid int, 
  effdt date,
  title varchar(100),
  primary key (emplid, effdt)
);

insert into job set emplid=1, effdt='2010-01-01', title='Programmer';
insert into job set emplid=1, effdt='2011-01-01', title='Programmer I';
insert into job set emplid=1, effdt='2012-01-01', title='Programmer II';

insert into job set emplid=2, effdt='2010-01-01', title='Analyst';
insert into job set emplid=2, effdt='2011-01-01', title='Analyst I';
insert into job set emplid=2, effdt='2012-01-01', title='Analyst II';

#Get each employees current job:
select *
from job a
where a.effdt=
    (select max(b.effdt) 
    from job b
    where b.emplid=a.emplid);

结果:

+--------+------------+---------------+
| emplid | effdt      | title         |
+--------+------------+---------------+
|      1 | 2012-01-01 | Programmer II |
|      2 | 2012-01-01 | Analyst II    |
+--------+------------+---------------+

我想将查询重写为一个没有子查询的连接。这可能吗？

Answer 1

将其写成join 可能有点违反直觉。这个想法是使用 left outer join 并在条件中包含 b.effdt > a.effdt。此条件将匹配行，除非 a.effdt 取最大值。然后查询可以使用 where 过滤这些:

select a.*
from job a left outer join
     job b
     on  b.emplid = a.emplid and
         b.effdt > a.effdt
where b.effdt is NULL;