NULL « Data Types « MySQL Tutorial

10.18.1.NULL means 'a missing unknown value'
10.18.2.Use the IS NULL and IS NOT NULL operators for NULL value
10.18.3.In MySQL, 0 or NULL means false and anything else means true.
10.18.4.Two NULL values are regarded as equal in a GROUP BY
10.18.5.NULL values are presented first if you do ORDER BY ... ASC
10.18.6.NULL values are presented last if you do ORDER BY ... DESC
10.18.7.Use 0 and '' is not NULL