Understanding Null Values

How does a database represent a value that is unknown? The answer is to use a special value, known as a null value. A null value is not a blank string—it is a distinct value. A null value means the value for the column is unknown.

When you select a column that contains a null value, you see nothing in that column. You saw this (or rather, didn’t see it!) in the earlier examples of selecting rows from the customers table: customer #4 has a null value in the dob column, and customer #5 has a null value in the phone column.