Question

Im trying to get all available keys, meaning keys which have been returned (return_date is not null), key hasn’t been lost (lost_key = false), and keys never been rented before (id is null). However when I run this I get 0 recirds. If I remove the id condition, I only get keys which have been rented before and all the keys which are not in the keyActivity table are ignored. Does anyone know what I am doing wrong?

SELECT a.key_id, a.drawer_num
FROM 
    keys AS a 
    left outer JOIN keyActivity AS b 
        ON a.key_id = b.key_id
WHERE 
    return_date is not null 
    and lost_key =false
    and id is null;

SOLUTION USED:

SELECT a.key_id, a.drawer_num
FROM keys AS a LEFT JOIN keyActivity AS b ON a.key_id = b.key_id
WHERE (b.return_date is not null 
and b.lost_key =false) 
OR b.id is null;

Answer 1

id may be a field in both tables, and if this is so, your condition id is null will be interpreted erroneously as the id of keys to be NULL, thus resulting in an empty result set.

It should work if you call it by its full name, i.e. b.id because you want to exclude any previous keyActivity.

However, I also believe you want to have both previous conditions connected by AND and in parentheses, and then the last condition connected by OR because previous key activity is okay as long as the key has been returned.

SELECT * FROM keys AS a LEFT OUTER JOIN keyActivity AS b ON a.key_id = b.key_id
  WHERE (b.return_date IS NOT NULL OR b.id IS NULL) AND a.lost_key = false;

EDIT: I rearranged the conditions, to also catch the case in which the clerk loses a key before ever handing it out.