Question

I’ve become stuck on particular problem of finding common items between tables. I have to works with these 3 tables – People can own have more than one home.

I want to find all people that have owned the same home as “John Doe”

Persons
    +------+------------------+----------+
    | id   | firsname         | lastname |
    +------+------------------+----------+
    |    1 | John             | Doe      |
    +------+------------------+----------+

Home
+------+------------------+-----------+
| h_id |     address      | year_built|
+------+------------------+-----------+
|    1 | 1233 SQL PL NW   | 1995      | 
+------+------------------+-----------+

Ownership
+-----------+------------------+
| person_id |     house_id     |
+-----------+------------------+
|    1      |         1        |
+-----------+------------------+

My current thoughts on what the query should go is that I check Persons.id matches the ownership.id then find the house.id and compare it two a some Person2.

Answer 1

Basically you need to go

persons -> 
    ownership -> 
       ownership that's not John Doe -> 
         back to persons again

This should do it

SELECT p2.id,
       p2.first_name,
       p2.last_name
FROM   persons p 
   INNER JOIN ownership o 
     ON p.id = o.person_id 
   INNER JOIN ownership o2 
     ON p.id <> o2.person_id 
        AND o.house_id = o2.house_id 
   INNER JOIN persons p2 
     ON o2.person_id = p2.id 
WHERE p.id = 1

as dtbarne notes the where clause assume you know the ID. If you want you change the WHERE clause to

WHERE p.first_name = 'John' and p.Last_name = 'Doe'

The problem there of course is if two records in person share the name you’ll get multiple results so you want to add House information to the SELECT clause to differentiate