How to avoid duplicate entries when saving in database in vb.net - sql-server

Question

I have a button but it can save duplicate entries i don't know how to correctly put a if not exist operator pls help..
cmd = New SqlCommand("INSERT INTO Students(Familyname,Firstname,Middlename,StudentID)VALUES('" & txtname.Text & "','" & txtfname.Text & "','" & txtmname.Text & "','" & txtid.Text & "')", cn)
cn.Open()
i = cmd.ExecuteNonQuery
cn.Close()
If txtname.Text <> "" Then
ElseIf i > 0 Then
MsgBox("Save Sucessfully!", MessageBoxIcon.Information, "Success")
showrecord()
clear()
Else
MsgBox("Save Failed!", MessageBoxIcon.Error, "Error")
End If

Answer 1

You can use NOT EXISTS to prevent duplicate insert:
Dim sql = "INSERT INTO Students(Familyname, Firstname, Middlename, StudentID) " & _
"VALUES(#FamilyName, #Firstname, #Middlename, #StudentID)" & _
"WHERE NOT EXISTS(SELECT 1 FROM Students WHERE StudentId = #StudentID)"
Using cn As New SqlConnection("Your connection string here")
Dim cmd As SqlCommand = New SqlCommand(sql, cn)
cmd.Parameters.Add("#FamilyName", SqlDbType.VarChar, 50).Value = txtname.Text
cmd.Parameters.Add("#Firstname", SqlDbType.VarChar, 50).Value = txtfname.Text
cmd.Parameters.Add("#Middlename", SqlDbType.VarChar, 50).Value = txtmname.Text
cmd.Parameters.Add("#StudentID", SqlDbType.VarChar, 50).Value = txtid.Text
Dim i = cmd.ExecuteNonQuery
End Using
You should always use parameterized queries to avoid SQL Injection attacks.
NOTE: Please apply appropriate field types.

Answer 2

Try this one :
cn.Open()
Dim intReturn as integer
Dim strSql as string = "Select * from Students where StudentID = #StudentID"
sqlcmd = new sqlcommand(strSql, cn)
With sqlcmd.parameters
.addwithvalue("#StudentID", ctype(txtid.text,string)
End with
intReturn = sqlcmd.ExecuteScalar
If(intReturn > 0)
cmd = New SqlCommand("INSERT INTO Students(Familyname,Firstname,Middlename,StudentID)VALUES('" & txtname.Text & "','" & txtfname.Text & "','" & txtmname.Text & "','" & txtid.Text & "')", cn)
i = cmd.ExecuteNonQuery
If txtname.Text <> "" Then
ElseIf i > 0 Then
MsgBox("Save Sucessfully!", MessageBoxIcon.Information, "Success")
showrecord()
clear()
Else
MsgBox("Save Failed!", MessageBoxIcon.Error, "Error")
End If
Else
MsgBox("Student Already Exist", MessageBoxIcon.Error, "Error")
End If
cn.Close()
And don't forget to make your StudentID field as Unique in your database.