Question

SELECT Id
FROM Container_A 
JOIN Container_B 
ON 
Container_A.Container_B_FK = Container_B.Id
ORDER BY Container_A.Id

此查询返回与Container_B相关的所有Container_A项的所有元素。问题是 - 如何只获得与每个Container_B项目相关的第一个（带有min Id）项目。

Answer 1

    SELECT top 1 *,min(Id) as minimum
    FROM Container_A 
    JOIN Container_B 
    ON 
    Container_A.Container_B_FK = Container_B.Id
group by Container_A.id,Container_B.id 
    ORDER BY minimum

Answer 2

SELECT 
    MIN(a.Id) AS a_id,
    b.Id AS b_id
FROM Container_A a
JOIN Container_B b
ON (Container_A.Container_B_FK = Container_B.Id)
GROUP BY b.Id
ORDER BY a_Id

Answer 3

SELECT Id
FROM Container_A 
 JOIN Container_B 
   ON Container_A.Container_B_FK = Container_B.Id
 ORDER BY Container_A.Id ASC
 LIMIT 0,1

Answer 4

这样的查询存在的问题是，“first”根本没有任何意义，除非您指定项目的顺序在组内。如果您想要最低的商品ID，那么您可以对容器b进行分组并使用min aggreagate从每个组中获取最低的商品ID：

select
  b.Id,
  min(a.Id) as A_Id
from
  Container_B b
  inner join Container_A a on a.Container_B_FK = b.Id
group by
  b.Id
order by
  b.Id

不是全部返回，而是仅返回每个连接表项的第一项

4 个答案: