ACC: How to Check for Duplicate Values in Primary Key Fields

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The following example uses the Customers form in the sample database Northwind.mdb in Microsoft Access 7.0 and 97 (or NWIND.MDB in Microsoft Access 2.0 or earlier). The Customers form is based on the Customers table; CustomerID is the primary key in the table.

NOTE: Microsoft Access 97 has an example of the following in the sample database Northwind.mdb. On the Customers form there is a macro on the BeforeUpdate event of the CustomerId field that verifies that the CustomerId is unique.

To force an immediate check for duplicate values, follow these steps:

NOTE: CustomerID is called Customer ID in Microsoft Access 2.0 or earlier.

1. Create the following new macro. Make sure to select Conditions on the View menu before you begin entering this macro.
   
   NOTE: In the following sample macro, an underscore (_) is used as a line-continuation character. Remove the underscore when re-creating this macro.
   

         MacroName   Condition                                Action
         ----------------------------------------------------------------
         IsItDup     DCount("[CustomerId]","Customers", _     MsgBox
                     "[CustomerId]=Forms![Customers]! _
                      [CustomerId]")<>0
                     ...                                      CancelEvent
   
         IsItDup Actions
         ---------------------------------
         MsgBox
            Message: Duplicate Customer ID
         CancelEvent
   						

2. Open the Customers form in Design view. Change the BeforeUpdate property of the CustomerID field as follows:
   

         Text Box: CustomerID
            BeforeUpdate: IsItDup
   						

After you make this change, adding a duplicate value in the CustomerID field results in an error message. The insertion point remains in the field so that you can change the value.