If you omit null or not null in the create table statement, Adaptive Server uses the null mode defined for the database (by default, NOT NULL). Use sp_dboption to set the allow nulls by default option to true.
You must make an entry in a column defined as NOT NULL; otherwise, Adaptive Server displays an error message. See ““Unknown” values: NULL”.
Defining columns as NULL provides a placeholder for data you may not yet know. For example, in the titles table, price, advance, royalty, and total_sales are set up to allow NULL.
However, title_id and title are not, because the lack of an entry in these columns would be meaningless and confusing. A price without a title makes no sense, whereas a title without a price simply means that the price has not been set yet or is not available.
In create table, use not null when the information in the column is critical to the meaning of the other columns.
