Question

我有一个包含userid和follow列的关注表，我有一个带有userid和username的用户表。

我希望能够选择关注说用户4的用户，但是在同一个选择查询中有一列可以调用followornot，如果用户4不跟随被选中的人，则该值为0;如果用户4跟随，则为1选定的用户。

我心中有这样的东西，但它没有按预期工作。

查询如下

SELECT
  u.username,
  u.userid,
  (case when following= '4'  then 1 else 0 end) as followornot
FROM user u inner JOIN followTable f ON f.userid = u.userid
WHERE f.following = '4'

样本记录：

用户

╔════════╦══════════╗
║ USERID ║ USERNAME ║
╠════════╬══════════╣
║      1 ║ john     ║
║      2 ║ jane     ║
║      3 ║ smith    ║
║      4 ║ paul     ║
╚════════╩══════════╝

FollowTable

╔════════╦═════════════╗
║ USERID ║ FOLLOWINGID ║
╠════════╬═════════════╣
║      1 ║           2 ║
║      4 ║           1 ║
║      3 ║           4 ║
║      1 ║           4 ║
╚════════╩═════════════╝

SQLFiddle Link

Answer 1

SELECT  u.*,
        (d.followingID IS NULL) followornot
FROM    user u
        INNER JOIN followTable f
            ON u.userID = f.userID
        LEFT JOIN followTable d
            ON  u.userid = d.followingID AND
                d.userID = 4
WHERE   f.followingid = 4

SQLFiddle Demo

输出

╔════════╦══════════╦═════════════╗
║ USERID ║ USERNAME ║ FOLLOWORNOT ║
╠════════╬══════════╬═════════════╣
║      1 ║ john     ║           0 ║
║      3 ║ smith    ║           1 ║
╚════════╩══════════╩═════════════╝

Answer 2

为什么你不这样做：

select u.id, f.key_user, key_follower FROM user u INNER JOIN is_following f ON u.id=f.key_user AND f.key_follower=4;

其中u.id是user-id，key_user是fk in is_following，key_follower是user_id后跟user-id。

MySQL查询跟随系统

2 个答案: