Question

我有以下查询

SELECT * FROM collection
WHERE id = 1 AND (
    privacy = 0
    OR (privacy = 1 AND
        EXISTS (SELECT * FROM collection_member WHERE collection_id = collection.id AND member_id = 1)
    )
)

这很好。但是我也想从SELECT * FROM collection_member WHERE collection_id = collection.id AND member_id = 1输出结果。使用联接实现不的最有效方法是什么？（编辑：可能是subquery encapsulation？）

Answer 1

这里是将subselect列为一列的版本。

这仍然是联接，但也许更接近您的需求。

SELECT C.* , 
(SELECT MAX(M.MyColumn) 
 FROM collection_member M
 WHERE collection_id = C.id 
 AND M.member_id = 1
 ) MyColumn
FROM collection C
WHERE C.id = 1 
AND (
    C.privacy = 0
    OR (privacy = 1 AND
        EXISTS (
        SELECT * FROM collection_member CM
        WHERE CM.collection_id = C.id AND CM.member_id = 1)
    )
)

这种方法相对于执行JOIN的唯一好处是，确保不加入多对多和重复计算会更容易一些。

上面显示的查询使用MAX以确保仅返回一行。这可能是您想要的行，也可能不是。

Answer 2

您无法“从exists子查询中获取值”。

您必须加入：

SELECT *
FROM collection
LEFT JOIN collection_member ON privacy = 0
  OR (privacy = 1
      AND collection_id = collection.id
      AND member_id = 1
  )
WHERE id = 1

当然，与您的查询不同，collection行将在collection_member中的每个匹配行中出现一次。您可以通过汇总所有collection_member值来解决此问题。

Answer 3

尝试一下

SELECT c.*, cm.*
FROM collection c
        Left Join collection_member cm on (cm.collection_id = c.collection.id and cm.member_id = 1)
WHERE case
        when c.privacy = 0 Then 1
        when c.privacy = 1 and cm.collection_id is not null Then 1
        else 0
      end = 1
and c.id = 1

从内部返回值存在不带联接的子查询

3 个答案: