Given two columns from Elements table
+--------+------+
| Name   | Year |
+--------+------+
| Au     | NULL |
| Be     | 1798 |
| Fr     | 1939 |
| Uus    | 2010 |
+--------+------+
how many records will the following query return?
SELECT * FROM Elements
WHERE Year NOT IN (SELECT Year FROM Elements)
Explanation
Inner sub-query will eliminate every possible record the outer query could have returned.

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