SQL tricky left join of three tables - all different keys - sql

Question

Originally I've been trying to merge users_role_change_log table with users_schedule_log. In the first I've got only user id, and in second there is only full name. So no clear hit.
I've tried to introduce third table which could tie those two together - users table. There is users id, but full name is splitted in two columns first_name and second_name. So that further complicates things. Not my design.
Was hoping that I could improvise in concating ad hoc first_name and last_name from main users table and passing it as a relevant parameter for left join.
SELECT r.*,CONCAT(first_name, ' ', last_name) AS u.name
FROM `users_role_change_log` r
LEFT JOIN users u ON r.id = u.id
LEFT JOIN users_schedule_log e ON u.name = e.name
But this doesn't work.
Any recommendation?
Edit:
#1 added table examples:
a) users_role_change_log
id user_id role time_changed
34 60 19 '2020-07-05 05:30:00'
b) users_schedule_log
id name shift_start shift_end
9 'John Doe' '2020-07-05 07:00:00' '2020-07-05 15:00:00'
c) users
id first_name last_name
60 'John' 'Doe'
d)what I want to end up with
a)user_id b)name a)role a)time_changed b)shift_start b)shift_end
60 'John Doe' 19 ...

Answer 1

You must start the joins from users and the left join users_role_change_log and users_schedule_log on the correct columns:
SELECT u.id, CONCAT(u.first_name, ' ', u.last_name) name,
r.role, r.time_changed, s.shift_start, s.shift_end
FROM users u
LEFT JOIN users_role_change_log r ON r.user_id = u.id
LEFT JOIN users_schedule_log s ON s.name = CONCAT(u.first_name, ' ', u.last_name)
See the demo.

Answer 2

You could try it this way:
SELECT *
FROM `users_role_change_log` r
LEFT JOIN users u ON r.id = u.id
LEFT JOIN users_schedule_log e ON CONCAT(u.first_name, ' ', u.last_name) = e.name
or this way:
WITH user_connector as (select CONCAT(first_name, ' ', last_name) as name, id from users)
SELECT *
FROM `users_role_change_log` r
LEFT JOIN user_connector u ON r.id = u.id
LEFT JOIN users_schedule_log e ON u.name = e.name
Of course you shoud change the asterisk to the fields you want to select.

Answer 3

You can use the subquery for this. Hope to help, my friend :))
create table users_role(id int, user_id int, role int, time_changed datetime)
create table users_schedule(id int, name varchar(20), shift_start datetime, shift_end datetime)
create table users(id int, first_name varchar(20), last_name varchar(20))
insert into users_role
values(34, 60, 19, '2020-07-05 05:30:00')
insert into users_schedule
values(9, 'John Doe', '2020-07-05 07:00:00', '2020-07-05 15:00:00')
insert into users
values(60, 'John', 'Doe')
-----
Select *
FROM (SELECT r.*, CONCAT(u.first_name, ' ', u.last_name) AS Name
FROM users_role r
LEFT JOIN users u ON r.user_id = u.id )as t
LEFT JOIN users_schedule e ON t.Name = e.name

Answer 4

Find the answer in this fiddle. You need to construct a subquery for your users table where you CONCAT the columns and then proceed with the JOIN.
You might need to consider the database schema, since the table users_schedule_log can use the ID of the user rather than his name (same name problem).
SELECT r.user_id, u.name, r.role, r.time_Changed, u.shift_start, u.shift_end
FROM users_role_change_log AS r
LEFT JOIN (SELECT id, CONCAT(first_name,' ',last_name) AS name FROM users)A
ON r.user_id = A.id
LEFT JOIN users_schedule_log AS u
ON u.name = A.name