Question

我有2张桌子

Table Users: 
UserID  |  Name

Table Cars: 
CarID | Car Name | FK_UserID

用户可以拥有超过1辆汽车。

我想加入每个只有1辆车的用户，而不是更多。

看过这里的其他线程，我尝试了以下内容：

Select users.UserID, users.name, carid
from Users
join cars
on users.UserID = 
    (
    select top 1 UserID
    from users
    where UserID = CarID
    )

但是每个用户仍然会返回超过1个匹配。

我做错了什么？

Answer 1

您可以使用ROW_NUMBER()功能

尝试以下操作

select userid, username, carname
from
(
Select users.UserID as userid, 
users.name as username, 
cars.carname as carname,
ROW_NUMBER() OVER(PARTITION BY users.UserID ORDER BY users.UserID) AS r
from Users
join cars
on users.UserID = cars.FK_UserID
) XXX
where r = 1;

Answer 2

with x as
(select row_number() over(partition by userid order by carid) as rn,
 * from cars)
select u.userid, x.carid,  x.carname 
from users u join x on x.userid = u.userid
where x.rn = 1;

这是使用row_number函数执行此操作的一种方法。

Answer 3

另一种方法

select u.UserID, 
    u.name, 
    (select TOP 1 carid 
     from cars c 
     where u.UserID = c.FK_UserID 
     order by carid) carid -- Could be ordered by anything
from Users u
 -- where only required if you only want users with cars
where exists (select * from car c where u.UserID = c.FK_UserID)

Answer 4

最好是做一个子查询并使用group-by来为每个用户只返回1个用户和一辆汽车。然后将其加入外部用户表。

以下是一个例子：

select *
from user_table u
    join   (
           select userid
                , max(carname)
           from cars
           group by userid

           ) x on x.userId = u.userId

或者你可以使用上面的row_number()例子，如果你想要一个特定的订单（无论是这个例子还是他们的诀窍）

如何仅加入第一行，忽略进一步的匹配

4 个答案: