SQL: SELECT where 2 columns from different tables are the same

Question

I need to work with a database that contains info about (former) Presidents. I need to check if there a presidents that have the same hobbies AND are married in the same year.

So a president can have multiple hobbies in pres_hob table. And the marriage year is in the pres_mar table, in the mar_year column.

I've tried to INNER JOIN the tables in SQLite where the hobby and mar_year are equal, except for the pres_name. This way the JOIN doesnt work ofcourse, which makes sense. Im kinda new to this..

Any help is appreciated

Answer 1

Here's one option with multiple joins :

select p1.pres_name, p2.pres_name, ph.hobby
from pres_mar p1
     join pres_mar p2 on p1.pres_name != p2.pres_name and p1.mar_year = p2.mar_year
     join pres_hob ph on p1.pres_name = ph.pres_name
     join pres_hob ph2 on p2.pres_name = ph2.pres_name and ph.hobby = ph2.hobby

---

And depending on your expected results, another option using exists :

select pm.pres_name, ph.hobby
from pres_mar pm 
     join pres_hob ph on pm.pres_name = ph.pres_name
where exists (
    select 1
    from pres_mar pm2
         join pres_hob ph2 on pm2.pres_name = ph2.pres_name
    where pm.pres_name != pm2.pres_name and
          ph.hobby = ph2.hobby
    )

Answer 2

That sounds like a terrible database schema, I'm assuming its for learning purposes, anyway, you could do something like

SELECT
  h.name,
  h.hobby,
  m.year
FROM
  pres_hob h,
  pres_mar m
WHERE
  h.hobby = 'tennis'
AND
  m.year = 2016

This would return 1 record for every president with a marriage year of 2016 , and a hobby of Tennis.