prevent duplicate entries to database

Question

I want to prevent duplicate entries to my inventory form using vb.net and MySQL as the database, here is my code:

 Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
    Dim myCommand As New MySqlCommand
    Dim conn As MySqlConnection
    Dim i As String
    conn = New MySqlConnection
    conn.ConnectionString = "server = localhost;username= root;password= a;database= secret"
    Try
        conn.Open()
    Catch mali As MySqlException
        MsgBox("connot establish connection")
    End Try

    Dim intReturn As Integer
    Dim strSql As String = " select * from personnel where pcode = @pcode"

    Dim sqlcmd As New MySqlCommand(strSql, conn)
    With sqlcmd.Parameters
        .AddWithValue("@pcode", CType(pcode.Text, String))
    End With

    intReturn = sqlcmd.ExecuteScalar

    If (intReturn > 0) Then
        cmd = New MySqlCommand("Insert into personnel values('" & pcode.Text & "','" & lname.Text & "','" & fname.Text & "','" & office.Text & "','" & designation.Text & "')")
        i = cmd.ExecuteNonQuery


        If pcode.Text <> "" Then
        ElseIf i > 0 Then
            MsgBox("Save Successfully!", MessageBoxIcon.Information, "Success")
            mrClean()
            ListView1.Tag = ""
            Call objLocker(False)
            Call LVWloader()
            Call calldaw()
        Else
            MsgBox("Save Failed!", MessageBoxIcon.Error, "Error!")
        End If
    Else
        MsgBox("Personnel ID Already Exist!", MessageBoxIcon.Error, "Error!")

    End If

end sub

i found this while i search for answer, but when i tried to run it, it does not read the insert command but rather it goes directly to the msbox "Personnel ID Already Exist" even if theres no thesame Personnel ID.

can someone check why it does not read the insert please,

my Database tables values:

pcode = primary key

lname = longtext

fname = longtext

office = longtext

designation = longtext

any help will be much appreciated, thanks,

Answer 1

Sorry to say this is the wrong approach.

Databases have a built in system to prevent data being duplicated. That's through primary keys or unique key constraints. In your case, you have already created a primary key. So there is absolutely no need for you to do that SELECT COUNT(*) query.

Instead, just directly insert into the table and catch the integrity error when the pcode already exists.

Try
    cmd = New MySqlCommand("Insert into personnel values('" & pcode.Text & "','" & lname.Text & "','" & fname.Text & "','" & office.Text & "','" & designation.Text & "')")

    i = cmd.ExecuteNonQuery


    If pcode.Text <> "" Then
    ElseIf i > 0 Then
        MsgBox("Save Successfully!", MessageBoxIcon.Information, "Success")
        mrClean()
        ListView1.Tag = ""
        Call objLocker(False)
        Call LVWloader()
        Call calldaw()
    Else
        MsgBox("Save Failed!", MessageBoxIcon.Error, "Error!")
    End If
Catch ex As MySqlException
    MsgBox("Personnel ID Already Exist!", MessageBoxIcon.Error, "Error!")
End Try

Please also refer to the MySQL Manual Page PRIMARY KEY and UNIQUE Index Constraints

Answer 2

found the answer, as what @e4c5 said, its a wrong approach, so I restructed my code and finally made it work, just want to share the answer maybe it will help others.

Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
Dim myCommand As New MySqlCommand
Dim conn As MySqlConnection
Dim i As String
conn = New MySqlConnection
conn.ConnectionString = "server = localhost;username= root;password= a;database= secret"
Try
    conn.Open()
Catch mali As MySqlException
    MsgBox("connot establish connection")
End Try
    Dim retval As String
    Select Button4.Tag
            Case "ADD"
    with myCommand
       .Connection = conn
       .CommandText = "Select pcode from personnel where pcode = '" & pcode.Text & "'"
        retval = .ExecuteScalar
    If retval Is Nothing Then
      .CommandText = "Insert into personnel values ('" & pcode.Text & "','" & lname.Text & "','" & fname.Text & "','" & office.text & "','" & designation.Text & "')"
      .ExecuteNonQuery()
  Else
  MsgBox("Personnel ID Already Exist!", MessageBoxIcon.Error, "Error")
 End If
End With
End Sub

Answer 3

There should be the way you:

1) write a Trigger before Insert, and check if there is any similar row exist.
2) Put Unique Index on columns