Allow Null Values in a Column

If you omit null or not null in the create table statement, SAP ASE 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, SAP ASE 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.

• Constraints and Rules Used with Null Values
  You cannot define a column to allow null values, and then override this definition with a constraint or a rule that prohibits null values. For example, if a column definition specifies NULL and a rule specifies:
• Defaults and Null Values
  You can use defaults, that is, values that are supplied automatically when no entry is made, with both NULL and NOT NULL columns.
• Nulls Require Variable-Length Datatypes
  Only columns with variable-length datatypes can store null values. When you create a NULL column with a fixed-length datatype, SAP ASE converts it to the corresponding variable-length datatype. SAP ASE does not inform you of the type change.
• text, unitext, and image Columns
  text, unitext, and image columns created with insert and NULL are not initialized and contain no value. They do not use storage space and cannot be accessed with readtext or writetext.