Efficient way to join multiple columns to the same column? - SQL - sql

Question

I have a bunch of tables combined together, which each have a column containing some form of user_id. This leads to 12 user_id columns in total.
I want to join each of these user_id columns with a user_id column in a mapping table in order to retrieve the username for each of these user ids.
So (assuming I have 5 user id columns),
Input:
My Combined Tables Result:
t1.user_id t2.user_id t3.user_id t4.user_id t5.user_id
1 2 3 4 5
Mapping Table:
user_id username
1 A
2 B
3 C
4 D
5 E
Output:
t1.username t2.username t3.username t4.username t5.usernamne
A B C D E
My code looks something like:
SELECT m1.username, m2.username, m3.username, m4.username, m5.username
FROM {join logic for 5 tables here}
JOIN mapping m1
ON t1.user_id = m1.user_id
JOIN mapping m2
ON t2.user_id = m2.user_id
JOIN mapping m3
ON t3.user_id = m3.user_id
JOIN mapping m4
ON t4.user_id = m4.user_id
JOIN mapping m5
ON t5.user_id = m5.user_id
I'm realizing this is extremely inefficient, especially for 12 columns which would mean 12 JOINs. Is there a better or faster way to do this? Thanks!

Answer 1

You might find it easier to use a correlated subquery for each username, especially where you have to implement many columns as it's easier to cut n paste!
Something like:
select
(select Username from Mapping m where m.UserId = t.UserId1) Username1,
(select Username from Mapping m where m.UserId = t.UserId2) Username2,
(select Username from Mapping m where m.UserId = t.UserId3) Username3 etc
from InputTable t