Tutorial by Examples

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