LEFT JOIN 与主查询的相对值

Question

我有两个 table：用户和友谊。例如：

用户 table:

id | user  | email   | website
1  | Igor  | i@o.com | stack.com
2  | Lucas | a@i.com | overflow.com
3  | John  | j@w.com | www.com

友谊 table（当status = 2表示他们是朋友时）：

id | friend1 | friend2 | status
1  | Igor    | John    | 2
2  | Lucas   | Igor    | 2
3  | John    | Lucas   | 2

然后我对 select 朋友（其中 $user 是当前登录的用户）执行以下操作：

SELECT friend1, friend2 
FROM friendship 
WHERE (friend1 = '$user' OR friend2 = '$user') AND status = 2

但是我也想selectuserstable中的用户数据，因此，我会这样做：

SELECT friend1, friend2 
FROM friendship 
LEFT JOIN users 
ON user = **friend** 
WHERE (friend1 = '$user' OR friend2 = '$user') AND status = 2

在上面的ON子句中，我在friend这个词里打上了星号，大家看看我的疑惑在哪里。我只想 select table users 中的数据而不是当前登录的用户（$user），我知道我应该使用验证该 CASE 等于登录用户，但我不知道在这种情况下该怎么做。

Answer 1

我想你可以使用

select * from user where 
user in (
    select friends1 from friendship where friend2='$user' and status=2
    union all
    select friends2 from friendship where friend1='$user' and status=2
)

我添加了左连接示例

select u.* from (
    select friends1 f from friendship where friend2='$user' and status=2
    union all
    select friends2 f from friendship where friend1='$user' and status=2) a
left join user u on a.f=u.user

Answer 2

这适用于您的样本数据（假设您需要好友中两个朋友的用户信息）：

SELECT f.friend1, u1.id, u1.email, u1.website, f.friend2, u2.id, u2.email, u2.website
FROM friendship f
INNER JOIN users u1
ON f.friend1 = u1.user 
INNER JOIN users u2
ON f.friend2 = u2.user
WHERE (friend1 = '$user' OR friend2 = '$user')
AND f.status = 2

Answer 3

这很容易通过简单的 JOIN 完成，其中只包含您的逻辑：

SELECT
    U.id,
    U.user,
    U.email,
    U.website
FROM
    Friendship F
INNER JOIN Users U ON
    (U.user = F.friend1 AND F.friend1 <> '$user') OR
    (U.user = F.friend2 AND F.friend2 <> '$user')
WHERE
    F.friend1 = '$user' OR F.friend2 = '$user'

LEFT JOIN 与主查询的相对值

LEFT JOIN with relative value from main query

mysql

sql

join

left-join