In most cases, columns in a table can contain null values. A null value means that the value for the column can be unknown or not applicable. For example, in the telephone directory example in Building a Relational Data Model, the anniv column of the name table can contain null values; if you do not know the person's anniversary, you do not specify it. Do not confuse null value with zero or a blank value. For example, the following statement inserts a row into the manufact table of the stores_demo database and specifies that the value for the lead_time column is null:
INSERT INTO manufact VALUES ('DRM', 'Drumm', NULL)
Collection columns cannot contain null elements. Creating and Using Extended Data Types in Dynamic Server describes collection data types.