# Uses for NULL
- Data not yet known - such as
- 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).
# Testing NULLs
IS NOT NULL--
= NULLdoes not work like you expect.
x <=> yis a "null-safe" comparison.
LEFT JOIN tests for rows of
a for which there is not a corresponding row in
SELECT ... FROM a LEFT JOIN b ON ... WHERE b.id IS NULL