NULL represents an UNKNOWN VALUE. It is
not an empty string, or a zero value. These are all valid values, and are not NULLs.
When a table is created or the format
altered , columns can be SPECIFIED as accepting NULL values, or not accepting them, with the NULL and NOT NULL clauses respectively.
For example, a customer table COULD contain dates of birth. For some customers, this information is unknown, so the value could be NULL.
The same system could allocate a customer ID for each customer record, and in this case a NULL value would not NULL represents an unknown value. It is
not an empty string, or a zero value. These are all valid values, and are not NULLs.
When a table is created or the format
altered , columns can be specified as accepting NULL values, or not accepting them, with the NULL and NOT NULL clauses respectively.
For example, a customer table could contain dates of birth. For some customers, this information is unknown, so the value could be NULL.
The same system could allocate a customer ID for each customer record, and in this case a NULL value would not be permitted.