mysql - 表 JOIN 没有像在 mysql 中应该的那样工作

Question

我有 2 个表，第一个是 users_table，第二个是 friends_table。

users_table 包含有关用户的信息，包括 banned 列，而 friends_table 是用户之间的友谊。

两个表都有 user_id 列，所以我想要的是根据两个表的条​​件检索特定用户的好友数量。条件是 user_id 和 banned。我希望 friend 的数量不包括被禁止的用户，但即使用户 friend 被禁止，我仍然会得到 friend 总数。请帮助我，这是我同时运行以检索 friend 的查询:

$query1 = "
    SELECT * 
    FROM users_table u 
    INNER JOIN friends_table f ON u.user_id = f.user_id 
    WHERE f.user_id = $user_id AND u.banned = 0
";

$query2 = "
    SELECT * 
    FROM users_table u 
    INNER JOIN friends_table f ON u.user_id = f.friend_id
    WHERE f.friend_id = $user_id AND u.banned = 0
";

编辑“用户表”结构:

user_id
first_name
last_name
username
email
password
temporarily_banned
banned

“friends_table”结构:

friendship_id
user_id
friend_id
friendship_timestamp

表记录示例

users_table 示例:

user_id: 20
first_name: Firas
last_name: Helou
username: firashelou
email: mac_987@hotmail.com
password: *******
temporarily_banned: 0
banned: 0

user_id: 30
first_name: Elie
last_name: Helou
username: eliehelou
email: elie@hotmail.com
password: *******
temporarily_banned: 0
banned: 1

user_id: 22
first_name: Jessy
last_name: Helou
username: jessyhelou
email: jessy@hotmail.com
password: *******
temporarily_banned: 0
banned: 1

user_id: 32
first_name: Jad
last_name: Helou
username: jadhelou
email: jad@hotmail.com
password: *******
temporarily_banned: 0
banned: 0

friends_table 示例:

friendship_id | user_id | friend_id | friendship_timestamp
10                20         30             1534342490
9                 20         22             1533484062
16                32         20             1541619611

Answer 1

您需要在user 表上添加一个JOIN 来检查 friend 是否被禁止。

您的第一个查询变为:

SELECT * 
FROM users_table u 
INNER JOIN friends_table f ON f.user_id = u.user_id
INNER JOIN users_table uf ON uf.user_id = f.friend_id AND uf.banned = 0
WHERE u.user_id = $user_id

对于您的第二个查询:

SELECT * 
FROM users_table u 
INNER JOIN friends_table f ON f.friend_id = u.user_id
INNER JOIN users_table uf ON uf.user_id = f.user_id AND uf.banned = 0
WHERE u.user_id = $user_id 

也可以将两个查询合并为一个，例如:

SELECT * 
FROM users_table u 
LEFT JOIN friends_table f1 ON f1.friend_id = u.user_id
LEFT JOIN users_table uf1 ON uf.user_id = f1.user_id AND uf1.banned = 0
LEFT JOIN friends_table f2 ON f2.user_id = u.user_id
LEFT JOIN users_table uf2 ON uf2.user_id = f2.friend_id AND uf2.banned = 0
WHERE u.user_id = $user_id AND COALESCE(uf1.id, uf2.id) IS NOT NULL

PS:如果您只是想查找好友的计数，您可以使用SELECT COUNT(*) 而不是SELECT *。