Access VBA/Prevent duplicate values - ms-access

Question

How I can prevent duplicate values to not insert into the table. I have created a code to INSERT, UPDATE and DELETE and I want to display a MsgBox that there is a duplicate value and to cancel it. Thanks. Below you have the code:
Private Sub Command12_Click()
If Me.emID.Tag & "" = "" Then
If (IsNull(Me.emID) Or (Me.emID = "") Or IsNull(Me.emFirst) Or (Me.emFirst = "") Or IsNull(Me.emLast) Or (Me.emLast = "")) Then
Me.emID.BorderColor = vbRed
Me.emFirst.BorderColor = vbRed
Me.emLast.BorderColor = vbRed
MsgBox "Please fill required fields", vbInformation, "Information"
Exit Sub
End If
CurrentDb.Execute "INSERT INTO tblEmployees(emID, first, last, gender, phone, mobphone, city, state, zip, adress, email, comment)" & _
"VALUES ('" & Me.emID & "', '" & Me.emFirst & "', '" & Me.emLast & "', '" & Me.emGender & "', '" & Me.emPhone & "', '" & Me.emMob & "', '" & Me.emCity & "', '" & Me.emState & "', '" & Me.emZip & "', '" & Me.emAdress & "', '" & Me.emEmail & "', '" & Me.emComment & "')"
MsgBox "Record Added", vbInformation, "information"
Else
CurrentDb.Execute "UPDATE tblEmployees " & _
"SET emiD =" & Me.emID & _
", first ='" & Me.emFirst & "'" & _
", last = '" & Me.emLast & "'" & _
", gender ='" & Me.emGender & "'" & _
", phone = '" & Me.emPhone & "'" & _
", mobphone ='" & Me.emMob & "'" & _
", city ='" & Me.emCity & "'" & _
", state ='" & Me.emState & "'" & _
", zip ='" & Me.emZip & "'" & _
", adress ='" & Me.emAdress & "'" & _
", email ='" & Me.emEmail & "'" & _
", comment ='" & Me.emComment & "'" & _
"WHERE emID =" & Me.emID.Tag
MsgBox "Updated!", vbInformation, "Information"
End If
Me.tblEmployees_subform.Form.Requery
End Sub

Answer 1

It sounds like you'd like to update an employee if one exists for the given ID otherwise you'd like to add a new employee. You can prevent adding duplicate employees by first trying to update an employee record with the given ID and if no records were updated only then do you add a new employee record.
Private Sub Command12_Click()
If (IsNull(Me.emID) Or (Me.emID = "") Or IsNull(Me.emFirst) Or (Me.emFirst = "") Or IsNull(Me.emLast) Or (Me.emLast = "")) Then
Me.emID.BorderColor = vbRed
Me.emFirst.BorderColor = vbRed
Me.emLast.BorderColor = vbRed
MsgBox "Please fill required fields", vbInformation, "Information"
Exit Sub
End If
' You must set CurrentDb to a variable otherwise the RecordsAffected
' property used later will be incorrect.
Dim db As DAO.Database
Set db = CurrentDb
' First try to update an existing employee.
db.Execute _
"UPDATE tblEmployees " & _
"SET first ='" & Me.emFirst & "', " & _
"last = '" & Me.emLast & "', " & _
"gender ='" & Me.emGender & "', " & _
"phone = '" & Me.emPhone & "', " & _
"mobphone ='" & Me.emMob & "', " & _
"city ='" & Me.emCity & "', " & _
"state ='" & Me.emState & "', " & _
"zip ='" & Me.emZip & "', " & _
"adress ='" & Me.emAdress & "', " & _
"email ='" & Me.emEmail & "', " & _
"comment ='" & Me.emComment & "'" & _
"WHERE emID =" & Me.emID.Tag & ";"
' If no records were affected by update then add a new employee.
If db.RecordsAffected = 0 Then
db.Execute _
"INSERT INTO tblEmployees(emID, first, last, gender, phone, mobphone, city, state, zip, adress, email, comment) " & _
"VALUES ('" & Me.emID & "', '" & Me.emFirst & "', '" & Me.emLast & "', '" & Me.emGender & "', '" & Me.emPhone & "', '" & Me.emMob & "', '" & Me.emCity & "', '" & Me.emState & "', '" & Me.emZip & "', '" & Me.emAdress & "', '" & Me.emEmail & "', '" & Me.emComment & "');"
MsgBox "Record Added", vbInformation, "Information"
Else
MsgBox "Updated!", vbInformation, "Information"
End If
Me.tblEmployees_subform.Form.Requery
End Sub
Note: In the update query I removed the update to the emID field since that is what the query is based on (in the WHERE clause). If the emID field is changing you won't be able to use the new emID value to find an employee record with the old emID value.
If you never want any duplicates I would also suggest that you add constraints to your database table to prevent duplicates, as suggested by Daniel Cook. I would also suggest looking into using parameterized queries instead of building SQL strings in VBA.

Answer 2

You can change your SQL to use IF EXISTS condition and insert only if the records does not already exists.
Your SQL may look like:
IF NOT EXISTS
(
SELECT ......
)
BEGIN
INSERT INTO tblEmployees ......<insert since employee does not exists>
END