List all zip codes that are not assigned to students. Write two versions: using a NOT EXISTS and using an OUTER JOIN.

What will be an ideal response?

```SELECT zip
FROM zipcode z
WHERE NOT EXISTS (SELECT null
FROM student s
WHERE s.zip=z.zip)

SELECT z.zip
FROM zipcode z, student s
WHERE z.zip = s.zip (+)
AND s.zip IS NULL

SELECT z.zip
FROM zipcode z LEFT OUTER JOIN student s
ON z.zip = s.zip
WHERE s.zip IS NULL```