• Data not yet known - such as end_date, rating
• Optional data - such as middle_initial (though that might be better as the empty string)
• 0/0 - The result of certain computations, such as zero divided by zero.
• NULL is not equal to "" (blank string) or 0 (in case of integer).
• others?

• IS NULL / IS NOT NULL -- = NULL does not work like you expect.
• x <=> y is a "null-safe" comparison.

In a LEFT JOIN tests for rows of a for which there is not a corresponding row in b.

SELECT ...
    FROM a
    LEFT JOIN b ON ...
    WHERE b.id IS NULL