The power of a relational database system such as Microsoft Access comes from its ability to quickly find and bring together information stored in separate tables using queries, forms, and reports. In order to do this, each table should include a field or set of fields that uniquely identifies each record stored in the table. This information is called the primary key of the table. Once you designate a primary key for a table, to ensure uniqueness, Microsoft Access will prevent any duplicate or Null values from being entered in the primary key fields.