Private Sub btnsave4_Click()
Dim SQLMatrikelnr As String
 
If dbRecCount(mConn, "Student", "MatrikelNr='txtmatrikelnr(0).Text'") > 0 Then
MsgBox "Die Nummer existiert bereits"
Else
SQLMatrikelnr = "INSERT INTO Student ([MatrikelNr],[...]) Values ('" & _
  txtmatrikelnr(0).Text & "','" & ... & "')"
mConn.Execute SQLMatrikelnr
End If

Public Function dbRecCount(mConn As Connection, ByVal sTable As String, _
  Optional ByVal sWHERE As String = "")
 
  Dim mRs As ADODB.Recordset
  On Error Resume Next
  Set mRs = mConn.Execute("SELECT Count(*) FROM [" & sTable & IIf(Len(sWHERE) _
    > 0, "] WHERE " & sWHERE, "]"))
  If Not mRs.EOF And Not mRs.BOF Then
    dbRecCount = mRs(0)
  End If
  mRs.Close
  Set mRs = Nothing
  On Error GoTo 0
End Function