Example of a data diagram: The arrows show the Record Source for the Forms and the Control Source for the Combo Box. Before You Begin: Draw A Data Diagram Each object in a database either looks up the information in a Table or modifies the information in the Table.   These lessons will build Forms with Subforms. A Subform is a Form that is added to another Form. The two Forms are linked by Fields that both share. This is how a relational database represents a one-to-many relationship.   Each Form and Subform will have a different Record Source, so it is important to keep track of which Table or Query goes with which Form or Report.   A data diagram helps organize your plans. It is a simplified sketch of what you would see when the Forms are in Design View.