Question

I usually go with the join approach but in this case I am a bit confused. I am not even sure that it is possible at all. I wonder if the following query can be converted to a left join query instead of the multiple select in used:

select 
   users.id, users.first_name, users.last_name, users.description, users.email 
from users 
where id in (
     select assigned.id_user from assigned where id_project in (
            select assigned.id_project from assigned where id_user = 1
                                                               )
            ) 
   or id in (
     select projects.id_user from projects where projects.id in (
            select assigned.id_project from assigned where id_user = 1
                                                                )
            )  

This query returns the correct result set. However, I guess the repetition of the query that selects assigned.id_project is a waste.

Answer 1

Since both subqueries have a condition where assigned.id_user = 1, I start with that query. Let’s call that assignment(s) the ‘leading assignment’.

Then join the rest, using left joins for the ‘optional’ tables.
Use an inner join on user that matches either users of assignments linked to the leading assignment or users of projects linked to the leading project.

I use distinct, because I assumen you’d want each user once, event if they have an assignment and a project (or multiple projects).

select distinct
  u.id, u.first_name, u.last_name, u.description, u.email 
from 
  assigned a
  left join assigned ap on ap.id_project = a.id_project
  left join projects p on p.id = a.id_project
  inner join users u on u.id = ap.id_user or u.id = p.id_user
where
  a.id_user = 1