Tables, Table Design

Relational databases support a special value in fields, called a Null, that indicates an unknown value. Nulls have special properties. A Null value cannot be equal to any other value, not even to another Null. This means you cannot join (link) two tables on Null values. Also, the test "A = B," when A, B, or both A and B contain a Null, can never yield a true result. Finally, Null values do not participate in aggregate calculations involving such functions as Sum or Avg. You can test a value to determine whether it is a Null by comparing it to the special keyword NULL or by using the IsNull built-in function.