SQL Left Join in a single table without duplicate - mysql

Question

I have a junction table with Users ids and projects ids :
Every user can be on multiple projects.
id|UserID |ProjectID|
1 | 1| 2 |
2 | 2| 3 |
3 | 1| 3 |
4 | 3| 4 |
5 | 3| 2 |
6 | 3| 3 |
7 | 4| 5 |
Is it possible to return, with a single sql request all the id of user who are in the same projects as me ?
For example here, return me :
user 3, because he is in the projects 2 and 3 with me
user 2, because he is in the project 3 with me
I tried tu use DISTINCT with LEFT JOIN, but i can't find how to use it with only one table...
Thanks

Answer 1

There are usually a few ways you can achieve something like this in sql. The first method is one that I would have written, just to show you an alternate way of thinking about it. I also wrote a left join as you were attempting to show you that method as well.
Method #1
SELECT DISTINCT UserID
FROM Table
WHERE ProjectID IN (SELECT DISTINCT ProjectID FROM Table WHERE UserID = me)
Method #2
SELECT DISTINCT others.UserID
FROM Table myself
LEFT JOIN Table others ON myself.ProjectID = others.ProjectID
WHERE myself.UserID = me

Answer 2

cant think of anything better for now
declare #searchingId int;
set #searchingId = 1
select distinct t2.UserId
from
(select t1.ProjectId
from [Table] as t1
where t1.UserId = #searchingId) as projects
join [Table] as t2 on projects.ProjectId = t2.ProjectId
where t2.UserId != #searchingId

Answer 3

I would use a simple join:
select distinct upr.userid
from userprojects upr join
userprojects upr2
on upr2.projectid = upr.projectid and
upr2.userid = #me;
If you want to, say, count the number of matching projects, use group by:
select upr.userid, count(*)
from userprojects upr join
userprojects upr2
on upr.projectid = upr.projectid and
upr2.userid = #me
group by upr.userid;