子查询左连接引用父 ID

Question

我正在尝试查询以获取每个用户的最新汽车：

select * from users 
    left join 
       (select cars.* from cars 
        where cars.userid=users.userid 
        order by cars.year desc limit 1) as cars
    on cars.userid=users.userid

where 子句中好像写着 Unknown column "users.userid" 我试图删除 cars.userid=users.userid 部分，但它只会获取 1 辆最新的汽车，并将其贴在每个用户身上。

有什么方法可以实现我想要的吗？谢谢！！

Answer 1

也许有更好更有效的查询方式。这是我的解决方案；

select users.userid, cars.*
from users
         left join cars on cars.userid = users.userid
         join (SELECT userid, MAX(year) AS maxDate
               FROM cars
               GROUP BY userid) as sub on cars.year = sub.maxDate;

Answer 2

一个选项是使用子查询过滤 left join：

select * -- better enumerate the columns here
from users u
left join cars c 
    on  c.userid = u.userid
    and c.year = (select max(c1.year) from cars c1 where c1.userid = c.userid)

为了提高性能，请考虑 car(userid, year) 上的索引。

请注意，如果您在 cars 中有重复的 (userid, year)，那么每个用户可能 return 多辆汽车。最好有一个真实的日期而不是只有年份。

Answer 3

为此，我通常使用row_number():

select *
from users u left join
     (select c.* , row_number() over (partition by c.userid order by c.year desc) as seqnum
      from cars c
     ) c 
     on c.userid = u.userid and c.seqnum = 1;

子查询左连接引用父 ID

Subquery left join refer to parent ID

mysql

sql

select

join

greatest-n-per-group