如何知道数据库中是否已存在用户名？

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

您可以在数据库中将用户名字段指定为主键。