primary key

A primary key is a column or set of columns that can be used to identify or access a particular row or rows. A primary key is a unique key; that is, it is a key that is constrained so that no two of its values are equal. A table cannot have more than one primary key, and the columns of a primary key cannot contain null values. To change a column to be nonnullable, use the Change Column Categories window from the Change Columns page.