If you specify no default value for a column, the default is NULL unless you place a NOT NULL constraint on the column. In this case, no default exists.
If you specify NULL as the default value for a column, you cannot specify a NOT NULL constraint as part of the column definition. (For details of NOT NULL constraints, see Using the NOT NULL Constraint.)
NULL is not a valid default value for a column that is part of a primary key.
If the column is a BYTE or TEXT data type, NULL is the only valid default value.
In Dynamic Server, if the column is a BLOB or CLOB data type, NULL is the only valid default value.
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