usernameが既にデータベースに存在するかどうかを知るには？

Question

Dim okToadd As MsgBoxResult = MsgBox("Are you sure you want to add the current record?", MsgBoxStyle.YesNo) 
    If okToadd = MsgBoxResult.Yes Then 
     MsgBox("Admin information Added Sucessfully", MsgBoxStyle.OkOnly) 

     provider = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source =" 
     'Change the following to your access database location 
     dataFile = "C:\Users\talha\Documents\login.accdb" 
     connString = provider & dataFile 
     myConnection.ConnectionString = connString 

     myConnection.Open() 
     Dim str As String 
     str = "insert into loginusers ([username], [password], [firstname], [lastname]) values (?, ?, ?, ?)" 
     Dim cmd As OleDbCommand = New OleDbCommand(str, myConnection) 


     cmd.Parameters.Add(New OleDbParameter("username", CType(TextBox1.Text, String))) 
     cmd.Parameters.Add(New OleDbParameter("password", CType(TextBox2.Text, String))) 
     cmd.Parameters.Add(New OleDbParameter("firstname", CType(TextBox3.Text, String))) 
     cmd.Parameters.Add(New OleDbParameter("lastname", CType(TextBox4.Text, String))) 







     Try 

      cmd.ExecuteNonQuery() 
      cmd.Dispose() 
      myConnection.Close() 
      TextBox1.Clear() 
      TextBox2.Clear() 
      TextBox3.Clear() 
      TextBox4.Clear() 
      DataGridView1.Refresh() 

     Catch ex As Exception 
      DataGridView1.Refresh() 
      MsgBox(ex.Message) 
      DataGridView1.Refresh() 
     End Try 
    ElseIf okToadd = MsgBoxResult.No Then 

    End If 

Answer 1

ユーザー名フィールドをデータベースの主キーとして指定できます。