NULL is a special value that matches missed data. It means there is no data inserted into the field.

There are operators to detect whether the value is NULL:

• IS NULL: detects only values that equal to NULL

• IS NOT NULL: detects values that are not NULL.

NULL does not match 0 or empty line, it only matches data that is missing and does not exist.

NULL values are ignored in the MIN, MAX, COUNT, AVG, and SUM aggregate functions.