98. IS NULL, IS NOT NULL, and Three-Valued Logic

In SQL, col = NULL is never TRUE — comparisons with NULL produce UNKNOWN, which the WHERE clause treats as not-true and filters out. To test for missing values you must use IS NULL / IS NOT NULL. This is the heart of three-valued logic (TRUE / FALSE / UNKNOWN).

Task: the tasks table has an assignee that may be NULL. Return id, title, and status where status is 'unassigned' when assignee IS NULL and 'assigned' otherwise. Then keep only rows that are actually assigned (assignee IS NOT NULL). Order by id.

Output columns, in order: id, title, status.