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Microsoft KB Archive/102527

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Knowledge Base

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

Article ID: 102527

Article Last Modified on 1/18/2007


  • Microsoft Access 1.0 Standard Edition
  • Microsoft Access 1.1 Standard Edition
  • Microsoft Access 2.0 Standard Edition
  • Microsoft Access 95 Standard Edition
  • Microsoft Access 97 Standard Edition

This article was previously published under Q102527

Moderate: Requires basic macro, coding, and interoperability skills.


When you are adding records, Microsoft Access does not normally check the values in primary key fields for duplicates until you move to the next record. If you enter an invalid or duplicate value in a control and move to the next record, you may invalidate all previous entries. However, there is a method to force an immediate check for duplicate values. This article describes how to check for duplicate values immediately.


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]! _
                      ...                                      CancelEvent
          IsItDup Actions
             Message: Duplicate Customer ID
  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.

Additional query words: count before update

Keywords: kbhowto kbusage KB102527