NULLs in arithmetic
If one or both of the operands of the arithmetic operators (+, -, *, /) is NULL,
the result is NULL.
NULLs and three-valued logic
NULL as either operand of a comparison (< <= = <> >= >)
will result in a unknown value as the result - not true, not false.
Inefficiency of NULLs
If a field can be NULL, it's necessary for the database to keep extra information
about each row for that column. Usually it is just one extra bit, but it has to
be tested each time.
General Advice: Avoid using NULLs
Clarifying NULL with an Extra Column
One solution to the multiple possible meanings of NULL is to use an extra column
which tells what that NULL means - missing data, not applicable, ....