For columns that allow NULL values, specifying a NULL default is exactly the same as not specifying a default at all. If the client inserting the row does not explicitly assign a value, the row automatically receives a NULL value.
You can use NULL defaults when information for some columns is optional or not always available.
For more information about the NULL value, see “NULL value” in Chapter 2, “SQL Language Elements,” of the Reference: Building Blocks, Tables, and Procedures.
