3. Using the Tables Utility : Create a Table : Column Specification--Nulls
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Column Specification--Nulls
This field allows you to specify whether the data column accepts null values. A null value is a special value that represents unknown or unavailable data. On standard installations, a null value appears to be an empty field, but there is a difference between a null value and a blank or a zero. A nullable column is one that accepts a null value. The Nulls field works in conjunction with the Defaults field.
To specify that the data column can accept null values, enter y in this field. If a user does not enter data into a nullable column, a value is entered for that column as specified by the Defaults field (see page Column Specification--Defaults).
To specify that the column cannot accept null values, enter n. When a user does not enter data into a non‑nullable data column, either the value of zero (0) is automatically entered into the column if it is a numeric column, or the column is left blank if it is a character column, or the user is required to enter a value. Use the Defaults column in the Column Specification form to specify this choice.