Sometimes you do not have enough information to enter values for every column in a row. SQL uses the keyword NULL to represent ‘unknown’ or ‘no value’. This is different from the meaning of NULL in programming languages such as C.
For example, if you are inserting a record for Mitchell into the table of composers and you do not know their address, you might execute the following:
INSERT INTO composers (id, name, address) VALUES (5, 'Mitchell', NULL);
If you do not specify the address field, it will contain NULL by default.
INSERT INTO composers (id, name) VALUES (5, 'Mitchell');
Note that NULL is not the same as zero or an empty string.
Unlike NULL, NOT NULL is one of the SQL data constraints. NOT NULL indicates that null values are not allowed in any row of the table for the specified column. For more information and examples, see SQL: Statements.