For each column, you can specify whether to allow null values. A null value is not the same as “zero” or “blank.” NULL means no entry has been made, and usually implies “value unknown” or “value not applicable.” It indicates that the user did not make any entry, for whatever reason. For example, a null entry in the price column of the titles table does not mean that the book is being given away free, but that the price is not known or has not yet been set.
If the user does not make an entry in a column defined with the keyword null, Adaptive Server supplies the value “NULL.” A column defined with the keyword null also accepts an explicit entry of NULL from the user, no matter what datatype it is. Be careful when you enter null values in character columns. If you put the word “null” inside single or double quotes, Adaptive Server interprets the entry as a character string rather than as the value NULL.
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.
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.
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