错误显示 MySQL 中使用 vb.net 的记录是否重复

Question

美好的一天！有人可以帮我修复我的代码吗？如果记录重复，我想显示一条错误消息。例如，我输入了一个用户名“admin”，但它已经在我的数据库中，所以它应该显示一条消息说“用户名已经存在！”。否则，如果用户名尚未使用，那么它将被添加到我的数据库中。我正在使用 Visual Studio 2005 和 Navicat for MySQL 这是我的代码：

conn.Open()

        Dim qadd As String = "SELECT * FROM tbl_user WHERE uname='" & txt_uname.Text & "'"
        Dim cmd As New MySqlCommand(qadd, conn)
        Dim data As MySqlDataReader = cmd.ExecuteReader

        If data.Read Then
            If data(0) = txt_uname.Text Then
                MsgBox("User " & data(0) & " already exists! ", MsgBoxStyle.Critical)
            Else
                Dim qstr As String = "INSERT INTO tbl_user (uname, pword, ulvl) VALUES ('" & txt_uname.Text & "' , '" & txt_pword1.Text & "' , '" & txt_pword2.Text & "') ON DUPLICATE KEY UPDATE uname = '" & txt_uname.Text & "'"
                Dim cm As New MySqlCommand(qstr, conn)
                Dim dat As MySqlDataReader = cm.ExecuteReader
                MsgBox("User has been added!", MsgBoxStyle.Information)
                txt_uname.Clear()
                txt_pword1.Clear()
                txt_pword2.Clear()
                txt_uname.Focus()
            End If
        End If

        conn.Close()

Answer 1

仍有很大的改进空间，我在 phone 上输入了这个，没有进行语法检查，但我认为它应该能让你朝着正确的方向前进。您需要阅读的内容是参数化您的 query/insert 语句和可以帮助管理数据库连接的 Using 关键字。

Dim qadd As String = "SELECT Count(uname)  FROM tbl_user WHERE uname='" & txt_uname.Text & "'"
Dim cmd As New MySqlCommand(qadd, conn)
Dim userCounter as int = cmd.ExecuteScaler
if userCounter > 0 then
  MsgBox("User " & data(0) & " already exists! ", MsgBoxStyle.Critical)
Else
  Dim qstr As String = "INSERT INTO tbl_user (uname, pword, ulvl) VALUES ('" & txt_uname.Text & "' , '" & txt_pword1.Text & "' , '" & txt_pword2.Text & "') ON DUPLICATE KEY UPDATE uname = '" & txt_uname.Text & "'"
  Dim cm As New MySqlCommand(qstr, conn)
  Dim dat As MySqlDataReader = cm.ExecuteReader
  MsgBox("User has been added!", MsgBoxStyle.Information)
  txt_uname.Clear()
  txt_pword1.Clear()
  txt_pword2.Clear()
  txt_uname.Focus()
End If

错误显示 MySQL 中使用 vb.net 的记录是否重复

Error show if there is a duplication of record in MySQL using vb.net

mysql

vb.net

navicat