How do I join 2 columns in MySql? - mysql

Question

I have the following table structure in my code and I am trying to pull username and name fields from users table, but the query currently pulls only from_user_id data. How do I modify this so that I get two separate columns that lists username and name for both to_user_id and from_user_id?
SELECT f.id, from_user_id, to_user_id, STATUS, u.username, u.name
FROM friend f
left JOIN users u ON f.from_user_id = u.id
WHERE f.id IN(
SELECT source_id
FROM notification
WHERE user_id = 5 AND notification_read = 1)
users table:
CREATE TABLE `users` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`username` VARCHAR(60) NOT NULL,
`password` VARCHAR(64) NOT NULL,
`enabled` TINYINT(4) NOT NULL DEFAULT '1',
`email` VARCHAR(100) NOT NULL,
`name` VARCHAR(100) NOT NULL,
`created_on` DATETIME NOT NULL,
`role` VARCHAR(50) NULL DEFAULT 'ROLE_USER',
PRIMARY KEY (`id`),
UNIQUE INDEX `username` (`username`)
)
and friend table:
CREATE TABLE `friend` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`from_user_id` BIGINT(20) NOT NULL,
`to_user_id` BIGINT(20) NOT NULL,
`status` INT(2) NOT NULL,
`requested_date` DATETIME NULL DEFAULT NULL,
`accepted_date` DATETIME NULL DEFAULT NULL,
PRIMARY KEY (`id`),
INDEX `from_user_id` (`from_user_id`),
INDEX `to_user_id` (`to_user_id`)
)
and a notification table:
CREATE TABLE `notification` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`user_id` BIGINT(20) NOT NULL,
`activity_type` TINYINT(4) NOT NULL,
`source_id` BIGINT(20) NOT NULL,
`parent_id` BIGINT(20) NULL DEFAULT NULL,
`parent_type` TINYINT(4) NULL DEFAULT NULL,
`notification_read` TINYINT(4) NOT NULL DEFAULT '0',
`created_on` DATETIME NOT NULL,
PRIMARY KEY (`id`),
INDEX `user_id` (`user_id`),
INDEX `created_on` (`created_on`)
)

Answer 1

You need to perform two joins against users - one for each side of the friend relationship, and include the appropriate columns in the SELECT list from both of those joins against users.
SELECT
f.id,
from_user_id,
to_user_id,
STATUS,
-- uf is an alias for the "from" user
-- You must alias the columns to distinguish them
uf.username AS from_username,
uf.name AS from_name,
-- ut is an alias for the "to" user
ut.username AS to_username,
ut.name AS to_name
FROM
friend f
-- Join first for the from user info
LEFT JOIN users uf ON f.from_user_id = uf.id
-- Join again for the to user info
LEFT JOIN users ut ON f.to_user_id = ut.id
WHERE f.id IN(
SELECT source_id
FROM notification
WHERE user_id = 5 AND notification_read = 1
)
A further note... You can substitute an INNER JOIN against notification instead of the IN () subquery, and you may achieve better performance.
SELECT
DISTINCT /* needed assuming multiple notification.source_id per f.id */
f.id,
from_user_id,
to_user_id,
STATUS,
uf.username AS from_username,
uf.name AS from_name,
ut.username AS to_username,
ut.name AS to_name
FROM
friend f
LEFT JOIN users uf ON f.from_user_id = uf.id
LEFT JOIN users ut ON f.to_user_id = ut.id
-- Join notification instead of the IN () subquery
INNER JOIN notification
ON f.id = notification.source_id
AND notification.user_id = 5
AND notification_read = 1