How to select column values based on the presence of a certain value in another column

Question

I have a list of projects, and different companies bidding for those projects.

project_name   company_name 
­___________________________     
    abc      |        1
    abc      |        2
    abc      |        3
    def      |        9
    def      |        7
    ghi      |        8
    jkl      |        1
    jkl      |        5
    jkl      |        6
    jkl      |        7

I want to keep only the projects where company "1" is present, while also keeping the other competitors for the project. The desired result is:

project_name   company_name 
­___________________________     
    abc      |        1
    abc      |        2
    abc      |        3
    jkl      |        1
    jkl      |        5
    jkl      |        6
    jkl      |        7

I have a feeling a subquery after WHERE is required to get that result, but I can't figure it out.

Answer 1

You can use exists :

select p.*
from projects p
where exists (select 1
              from projects p2
              where p2.project_name = p.project_name and
                    p2.company_name = 1
             );

Answer 2

Another option is window functions , often more efficient as it only requires one scan of the table. We can use a conditional count here:

select p.*
from (
    select *,
        count(case when company_name = 1 then 1 end) over (partition by project_name) hasCompany1
    from projects
) p
where hasCompany1 > 0;

Answer 3

This is the simplest query should work for this case:

select p.*
from projects p
where p.project_name in (select project_name from projects where company_name = 1)