NULL means unknown.

If NULL figures in an expression, most of the time the entire expression becomes NULL.

In aggregate functions only non-NULL fields are involved in the computation. Exception: COUNT(*).

UDFs sometimes convert NULL <-> non-NULL in a seemingly random manner.

If the test expression of an if statement is NULL, the then block is skipped and the else block executed.

To find out if A is NULL, use “A is (not) null”.

The COALESCE and *NVL functions can convert NULL to a value.

Assigning NULL is done like assigning values: with “A = NULL” or an insert list.