title: SQL : select unmatching records from 2 tables
body: <p>I have 2 tables and I want to retrieve the rows from the first table where the id_apartment does not appear in the second table:</p>
  
  <pre><code>id | id_floor | id_apartment 
  ----+----------+--------------
    1 |        0 |          101
    2 |        1 |          101
    3 |        1 |          102
    4 |        1 |          103
    5 |        1 |          104
    6 |        2 |          201
    7 |        2 |          202
    8 |        2 |          203
  
  
  table2.id | table2.guest | table2.apartment_id
  ----+---------------+--------------
    1 |         65652 |          101
    2 |         65653 |          101
    3 |         65654 |          101
    4 |         65655 |          101
    5 |         65659 |          102
    6 |         65656 |          201
    7 |         65660 |          202
    8 |         65661 |          202
    9 |         65662 |          202
   10 |         65663 |          203
  </code></pre>
  
  <p>expected output: </p>
  
  <pre><code>floor | number
  -------+--------
       1 |    103
       1 |    104
  </code></pre>
  
  <p>I tried using LEFT, INNER and RIGHT join but I always get EMPTY results. How can I manage this?</p>