Warning of duplicates in MS-Access - ms-access

Question

I am working on a MS-Access database to keep track of last names and addresses and several other records. I am making a table and would like for it to be updated by the user by inputting info from a form. The question I have is can I have several key items that would warn if a duplicate is found. Mainly if a duplicate is found in the last name and the address fields. I need the form to accept the duplicate because how the info is being obtained duplicates are possible but need to be able to enter the duplicate and warn the user while entering the data that a previous entry has already been made that is similar.
Is this possible in Access, and easy to implement that many different users may be entering the data? I haven't used Access for some time, sorry if I'm not using the correct terms, as I'm still learning the basics of Access, as I mainly work in Excel. Excel is the current solution that is being used, but finding quickly that a better solution is needed.
Thanks

Answer 1

Names aren't good candidates for primary keys and it seems that you have set the name as the primary key of one of your tables. You should either use a better candidate key (social insurance number, for example) or create a surrogate (artificial, if you want) key with no meaning other than being a key for your table.
Without more detail about your table structure it's hard to be more specific.

Answer 2

Warn the user but allow her to proceed if she wishes?
If so, a key constraint would be too, ahem, constraining. Just check for the duplicates manually (by explicitly querying existing data from your code). You may wish to index the field(s) being searched.

Answer 3

Yes, you can validate through this code:
If Nz(DCount("FieldName", "table", "FieldName= '" & Me.FieldName.Value & "'"), 0) > 0 Then
If MsgBox("The FieldName already exist", vbOKCancel, "Duplicate Warning") = vbCancel Then Cancel = True
Me.Undo
Exit Sub
End If
End If

Answer 4

In general terms, Access (and relational databases generally) are about preventing duplicate data from being entered in the first place. If you merely want to 'warn', then it will take a bit of custom code. So, say the table is called Customers and it has fields such as CustomerID (AutoNumber, primary key) and LastName (Text). The form then has a text box called txtLastName that is bound to the LastName field. In this situation, you could handle the text box's BeforeUpdate event like this:
Option Compare Database
Option Explicit
Const DuplicateWarningTitle = "Duplicate Record"
Const DuplicateValueWarning = "A record already exists with " + _
"the value you have entered. Are you sure you want to continue?"
Function GetSQLLiteral(Value As Variant) As String
Select Case VarType(Value)
Case vbNull
GetSQLLiteral = "Null"
Case vbString
GetSQLLiteral = "'" + Replace(Value, "'", "''") + "'"
Case Else
GetSQLLiteral = CStr(Value)
End Select
End Function
Sub HandleKeyFieldBeforeUpdate(Control As Access.TextBox, Cancel As Integer)
Dim RS As DAO.Recordset
Set RS = CurrentDb.OpenRecordset( _
"SELECT 1 FROM Customers " + _
"WHERE " + Control.ControlSource + " = " + GetSQLLiteral(txtLastName.Value) + _
" AND CustomerID <> " & Me.CustomerID)
If Not RS.EOF Then
Select Case MsgBox(DuplicateValueWarning, vbExclamation + vbYesNo, DuplicateWarningTitle)
Case vbYes
' just proceed
Case vbNo
Cancel = True
Control.Undo ' if desired!
End Select
End If
End Sub
Private Sub txtLastName_BeforeUpdate(Cancel As Integer)
HandleKeyFieldBeforeUpdate txtLastName, Cancel
End Sub
By pushing the duplicate checking code into a generic helper routine, other key fields can be handled in the same way easily:
Private Sub txtAddress_BeforeUpdate(Cancel As Integer)
HandleKeyFieldBeforeUpdate txtAddress, Cancel
End Sub