Know Your NULLs

These questions were inspired by the talk WAT that makes fun of some Javascript and Ruby "features". I've been reading The Art of Postgresql and thought SQL NULLs deserved a similar treatment. Please enjoy this quiz that sheds some light on the best NULL semantics in SQL. All examples were run in Postgres. Other RDBMSs may differ slightly.

employees
+----+-------+------------+
| id | name  | manager_id |
+----+-------+------------+
| 1  | Alice | NULL       |
| 2  | Bob   | 1          |
| 3  | Cara  | 2          |
| 4  | Dana  | NULL       |
+----+-------+------------+

SELECT id, name FROM employees WHERE manager_id != 1;