mysql - SQL - 连接两个表

Question

我有两张桌子

用户：（id，姓名）

关系：（user_id，relation_id）

User_id 和 relation_id 都是表中的 id users。

我想要的是恢复与特定用户成为朋友的所有用户。

这是我的 sql 命令：这不起作用：

SELECT *
FROM users
NATURAL JOIN relations
WHERE user_id IN (SELECT id FROM users WHERE name='John doe');

你可以帮帮我吗？

Answer 1

SELECT users.*
FROM relations
INNER JOIN users
  ON relations.relation_id = users.id
WHERE relations.user_id = 12345;

您还可以使用子查询获取 id，就像您在示例中所做的那样：

WHERE relations.user_id IN (SELECT id FROM users WHERE name='John doe');

Answer 2

要获取一个人拥有的所有关系，以下查询将起作用..

SELECT * FROM users us JOIN 关系 re ON us.id = re.relation_id WHERE re.user_id = ( SELECT id FROM users WHERE name = 'John doe' )

Answer 3

试试这个

SELECT * FROM users as a 
JOIN relations as b on a.id = b.user_id 
WHERE b.user_id IN (SELECT id FROM users WHERE name='John doe')

Answer 4

SELECT user_id
FROM relations re
JOIN users us
ON us.id = re.user_id
WHERE relation_id = (
    SELECT id
    FROM users
    WHERE name = 'John Doe'
)

旁注：您不能NATURAL JOIN在此处使用，因为在两个表中没有具有相同名称和类型的列。

Answer 5

这不只是通过您要查找的 userId 查询关系表的问题吗？

select * 
from relations
where  user_id = @IdYouArelookingFor or relation_id = @IdYouArelookingFor

Answer 6

SELECT friend.*
FROM users AS friend
  JOIN relations AS r
    ON r.relation_id = friend.id
  JOIN users AS u 
    ON u.id = r.user_id
WHERE u.name = 'John doe' ;

Answer 7

朋友是具有相同relation_id的人吗？

SELECT a.name FROM users a JOIN relations b on a.id = b.user_id
WHERE b.relation_id in 
(select relation_id from relations where id = 'userid of user you are looking for') 
AND a.id !=  'userid of user you are looking for'

如果您的逻辑不同，请告诉它是如何工作的