How Microsoft Access links main forms and subforms

You usually use a subform to display data from tables or queries that have a one-to-many relationship. For example, a Categories table and a Products table have a one-to-many relationship — each category can have more than one product.

When you create a form and subform based on tables that have a one-to-many relationship, the main form shows the "one" side of the relationship and the subform shows the "many" side of the relationship. The main form is synchronized with the subform so that the subform shows only records related to the record in the main form. If you use a wizard to create a subform, or if you drag a form, table, or query from the Database window to another form to create a subform, Microsoft Access automatically synchronizes the main form with the subform if both of the following are true:

• You have defined relationships for the tables you select, or you have defined relationships for the tables underlying the queries you select.
  
  
• The main form is based on a table with a primary key and the subform is based on a table that contains a field with the same name as that primary key and with the same or a compatible data type and field size. For example, if the primary key of the table underlying the main form is an AutoNumber field and its FieldSize property is set to Long Integer, the corresponding field in the table underlying the subform should be a Number field with its FieldSize property set to Long Integer. If you select a query or queries, the underlying tables for the query or queries must meet these conditions.

Note   Microsoft Access uses the LinkMasterFields and LinkChildFields properties of the subform control to link the main form and subform. If, for some reason, Microsoft Access doesn't link the form and subform, you can link the forms yourself.