MySQL從同一個表聯接多個列

Question

我正在嘗試編寫一個查詢，該查詢返回用戶個人資料信息以及另一個表中2列中用戶ID的出現次數。 下面的例子：

TableA
userID  userName
1       UserA
2       UserB

TableB
LinkID  leadID  followID
1       1       2
2       1       3
3       2       1

針對UserID 1的查詢我希望檢索UserA ， 2 （leadID中1的出現）和1 （在followID中1的出現）。 非常感謝您的任何幫助，並在此先感謝。

Answer 1

您實際上不需要為此而加入-您可以進行三個單獨的選擇。

SELECT
(
    SELECT userName
    FROM TableA
    WHERE userID = 1
) AS userName,
(
    SELECT COUNT(*)
    FROM TableB
    WHERE leadID = 1
) AS count_leadID,
(
    SELECT COUNT(*)
    FROM TableB
    WHERE followID = 1
) AS count_followID

結果：

userName  count_leadID  count_followID
UserA     2             1

Answer 2

SELECT a.userName,
       SUM(IF(b.leadId = a.userId, 1, 0) as Leads,
       SUM(IF(b.followId = a.userId, 1, 0) as Follows
FROM TableA a
     TableB b
GROUP BY a.userName

Answer 3

SELECT a.userName,
       b1.count(*) as leads,
       b2.count(*) as follows
FROM TableA a
INNER JOIN TableB b1 on a.userID = b1.leadID
INNER JOIN TableB b2 on a.userID = b2.followID
GROUP BY a.userName

根據mySQL的優化方式以及是否在leadID和followID上具有索引，這可以加快查詢的速度，特別是如果您要查詢幾個用戶而不是同時查詢所有用戶時。