phone number is not known @w@ have no phone, and thus no phone number

http://dev.mysql.com/doc/refman/5.7/en/problems-with-null.html

B.5.4.3 Problems with NULL Values

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ''. This is not the case. For example, the following statements are completely different:

mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ('');

Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as “phone number is not known” and the meaning of the second can be regarded as “the person is known to have no phone, and thus no phone number.”

http://dev.mysql.com/doc/refman/5.7/en/working-with-null.html

NULL means “a missing unknown value”.