Category: ASE

NULL Properties, Nullable Columns

NULL defines the columns of a table which can contain NULL values. This is also called nullabilty of a column. These columns are then called nullable columns.

Practical Meaning of Nullable Columns

If a new data set is inserted into a table, and the inserted data does not declare any value for a column which has been defined with a NULL attribute, then this data field is set to NULL.

If a column was defined with the attribute "NOT NULL", a value must be provided in insert or update statements. If this is not the case an error message will be returned. The same is true if no attributes have been defined for columns.

Usage of NULL When Creating Tables

Columns of tables can be declared with NULL if the future values are unknown and will be inserted at a later point in time. Columns should be declared with NULL only if essentially necessary. It is preferable to declare a default value.

Example for a NULL Property Column In the following example, a table with two columns will be created. The column column_name_a NULL values can be inserted, but not in column column_name_b.

    create table your_table

      (

         column_name_a int NULL

        , column_name_b int NOT NULL

      )

create table.