prevent duplicate entries to database - mysql

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

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

Answer 3

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