Simple Querying of a Table

Question

I have a table called residents that includes information about residents living in different apartment complexes. This is what selecting all values from this table would look like -

SELECT * FROM RESIDENTS;

 aptid   | aptnum          | ssn             
---------+-----------------+----------------
     100 |               1 |      641072301 
     100 |               2 |      699021306
     100 |               3 |      699021306     
     100 |               1 |      699021306       
     200 |               1 |      132429609 
     200 |               1 |      641072301
     300 |               1 |      641072301  

I want to select the social security numbers of all of the residents who are renting out more than three apartment in DIFFERENT complexes. So for example, resident with the ssn '641072301' will show up in the query because this person is renting out at least three units in complex ID 100, 200, 300 , where as the person with the social of '699021306' will not show up, because, even though they are renting out three different apartments, they are all within the same unit (aptID). so the result in this case would be -

 ssn             
---------+
641072301 

(1 row)

This is what I have, but it is no giving me what I want -

SELECT r1.ssn 
  FROM Residents r1
     , Residents r2
 WHERE r1.aptID <> r2.apt.ID;

Answer 1

Try this:

SELECT ssn 
FROM Residents
GROUP BY ssn 
HAVING COUNT(DISTINCT aptid) >= 3