如何在一个查询中使用两个 LEFT JOIN？

Question

我有两个查询，但我希望结果在同一个返回表中。

第一个查询返回所有帖子的列表，其中包含它们的 cmets 作为列表。 cmets 存储在不同的表中。

SELECT E.id, E.time, E.title, E.body, E.type, C.comments
FROM elements E
LEFT JOIN(
    SELECT elementID, GROUP_CONCAT(body SEPARATOR '|-|') AS comments
    FROM comments
    GROUP BY elementID
) C on C.elementID = E.id

第二个查询返回所有帖子的列表，其中包含帖子有多少喜欢/赞成的计数作为新列。喜欢的内容存储在不同的表中。

SELECT E.id, E.googleID, E.title, L.likeCount
FROM elements E
LEFT JOIN (
    SELECT elementID, COUNT(id) AS likeCount
    FROM likes
    GROUP BY elementID
) L ON L.elementID = E.id;

如何在同一个查询中将 cmets 的串联列表作为一列，并将喜欢的数量作为一列。

Answer 1

那么，你为什么不能像你已经为其他查询所做的那样JOIN 两个查询

SELECT E.id, E.time, E.title, E.body, E.type, C.comments, E.googleID, L.likeCount
FROM elements E
LEFT JOIN(
    SELECT elementID, GROUP_CONCAT(body SEPARATOR '|-|') AS comments
    FROM comments
    GROUP BY elementID
) C on C.elementID = E.id
LEFT JOIN (
    SELECT elementID, COUNT(id) AS likeCount
    FROM likes
    GROUP BY elementID
) L ON L.elementID = E.id;

Answer 2

您可以将同一级别的连接“堆叠”在一起，如下所示：

SELECT
    E.id
,   E.time
,   E.title
,   E.googleID
,   E.body
,   E.type
,   C.comments
,   L.likeCount
FROM elements E
LEFT JOIN(
    SELECT elementID, GROUP_CONCAT(body SEPARATOR '|-|') AS comments
    FROM comments
    GROUP BY elementID
) C on C.elementID = E.id
LEFT JOIN (
    SELECT elementID, COUNT(id) AS likeCount
    FROM likes
    GROUP BY elementID
) L ON L.elementID = E.id;

只需将要从两个连接返回的一组列放入选择列表中，然后将连接彼此相邻放置。

Answer 3

SELECT E.id, E.time, E.title, E.body, E.type,
(SELECT GROUP_CONCAT(body SEPARATOR '|-|') AS comments
FROM comment c where C.elementID= E.id group by C.elementID) as comments,
(SELECT  COUNT(id)
FROM likes L where L.elementID=E.id
GROUP BY elementID) as likeCount
FROM elements E