SQL - can I do repeated inner joins again on the same column? - mysql

Question

I have two simple tables, one called itineraries that holds details of holiday itineraries and one called users, that holds details of users. Other users create itineraries, and users can copy their itineraries and add travel agents, so the copied_from_id is the ID of the original creating user from users.id.
I've joined itineraries.user_id to users.id using the below query which works perfectly:
SELECT
itineraries.travel_agent_id,
itineraries.copied_from_id,
itineraries.user_id,
users.full_name,
users.username
FROM `gadabouting_gadabouting_production`.`itineraries`
INNER JOIN `gadabouting_gadabouting_production`.`users` ON itineraries.user_id=users.id
WHERE itineraries.travel_agent_id='253'
Giving me the following output:
+-----------------+------------------+---------+-------------+-------------+
| travel_agent_id | original_creator | user_id | full_name | username |
| 253 | 501 | 1465 | John Smithy | j.smithy |
| 253 | 501 | 1465 | John Smithy | j.smithy |
| 253 | 501 | 1474 | Ben Stockes | ben.stockes |
+-----------------+------------------+---------+-------------+-------------+
(The travel_agent_id and original_creator columns are the same as users.id).
What I want to do now is map the itineraries.travel_agent_id and itineraries.original creator to the users.full_name and users.username columns (so have the full_name and username columns printed next to each of the travel_agent_id and original_creator columns, but I just can't work out how to do it. I've spent hours on it now and can't get my head round it. Do I need to do more joins?
I've looked at several other SO questions about multiple joins but as far as I can see, none of them cover the process of 'going back' again and again on the same column as I want to do here.
Is this possible? Would greatly appreciate any help!
Thanks

Answer 1

You can join in the same table over and over, but you need to use an alias for each one so that you can specify which one you want to use. If you access the result by name, you also need alases for some of the field names.
(It's conventient to use aliases on other tables also, to make the query less verbose.)
select
i.travel_agent_id,
i.copied_from_id,
i.user_id,
u.full_name,
u.username,
ut.full_name as travel_agent_full_name,
ut.username as travel_agent_username,
uc.full_name as creator_full_name,
uc.username as creator_username
from
gadabouting_gadabouting_production.itineraries as i
inner join gadabouting_gadabouting_production.users as u on u.id = i.user_id
inner join gadabouting_gadabouting_production.users as ut on u.id = i.travel_agent_id
inner join gadabouting_gadabouting_production.users as uc on u.id = i.original_creator
where
i.travel_agent_id = '253'