MySQL通过两个表中的一个查询获取关系用户

Question

Im trying to get list of "friends" from my tables. I have a two tables, one where I store users list, and second one where I store relations with this users. I add on sqlfiddle example schemas. I need to fetch all "friends" for specified user id:

CREATE TABLE tbl_users
    (`id` int, `name` varchar(7), `description` varchar(55));

INSERT INTO tbl_users
    (`id`, `name`, `description`)
VALUES
    (1, 'John', '111'),
    (2, 'Derrik', '222'),
    (3, 'Nikolas', '333'),
    (4, 'Hellen', '444'),
    (5, 'Jack', '555');

CREATE TABLE tbl_relations
    (`id` int, `user_id` int, `following_id` int);

INSERT INTO tbl_relations
    (`id`, `user_id`, `following_id`)
VALUES
    (1, '1', '2'),
    (2, '1', '3'),
    (3, '1', '4'),
    (4, '2', '1'),
    (5, '4', '1');

Yes, I can do two queries and then compare it, but I thing this should work with one.

Result expected: if specified user id = 1, result should be get 2,4 user

Thx.

Answer 1

If the table

1. users
2. relations

The two tables must have columns that have the same value. If the columns is user_id so

SELECT * from users,relations WHERE users.user_id = relations.user_id;

I hope this answer can help you :)

Answer 2

try this:

SELECT tbl_users.* 

FROM tbl_users

INNER JOIN tbl_relations ON tbl_relations.user_id = tbl_users.id

More about join

Answer 3

"Result expected: if specified user id = 1, result should be get 2,4 user"

SELECT x.following_id reciprocations
  FROM tbl_relations x 
  JOIN tbl_relations y 
    ON y.user_id = x.following_id 
   AND y.following_id = x.user_id 
 WHERE x.user_id = 1;

Answer 4

This should return 2,3 for user id 1

SELECT id FROM tbl_relations 
WHERE user_id = 1 AND id != user_id;

To get the friends instead of their ids use

SELECT * from tbl_users WHERE id IN (SELECT id FROM tbl_relations 
WHERE user_id = 1 AND id != user_id);