Mysql 左连接与双同表关系 where 子句

Question

Basically I have these 2 simple mysql tables:

CREATE TABLE `users` (
  `id` int(10) UNSIGNED NOT NULL,
  `name` varchar(255) NOT NULL,
  `relation` int(10) UNSIGNED DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE `mytable` (
  `id` int(10) UNSIGNED NOT NULL,
  `user` int(10) UNSIGNED NOT NULL,
  `data` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,
  `tier` tinyint(3) UNSIGNED NOT NULL,
  `parent_id` int(10) UNSIGNED DEFAULT NULL COMMENT 'same table'
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `users` (`id`, `name`, `relation`) VALUES
(12, 'user1', 5),
(20, 'user2', NULL),
(21, 'user3', NULL);

INSERT INTO `mytable` (`id`, `user`, `data`, `tier`, `parent_id`) VALUES
(1, 12, 'test1', 1, NULL),
(2, 20, 'test2', 2, 1),
(3, 21, 'test3', 3, 1),
(4, 20, 'test4', 1, NULL);

I need to select all rows from 'mytable' that are related with the users.relation = 5

To do that I use the following query:

SELECT mytable.*
FROM mytable
    LEFT JOIN users ON mytable.user = users.id
WHERE users.relation = 5

The problem is that I also need to select the rows from mytable that have a parent_id connected with the above results.

WHERE mytable.id = X OR mytable.parent_id = X

Basically in this example, the expected output should be all rows of mytable except the last one (because is not connected with users.relation = 5)

Answer 1

You need another join of mytable to your query:

SELECT m.* 
FROM mytable m INNER JOIN (
  SELECT m.* 
  FROM mytable m 
  INNER JOIN users u ON m.user = u.id 
  WHERE u.relation = 5
) t ON t.id IN (m.id, m.parent_id)

I changed the join to INNER join, because although you use LEFT join the WHERE clause changes it to INNER since it filters out any unmatched rows.

See the demo .
Results:

> id | user | data  | tier | parent_id
> -: | ---: | :---- | ---: | --------:
>  1 |   12 | test1 |    1 |      null
>  2 |   20 | test2 |    2 |         1
>  3 |   21 | test3 |    3 |         1