mysql - 在调用Read()INSERT之前尝试访问字段无效

我试图用这个代码检查系统是否已经存在一个具有这个值的字段

Dim adap As New MySqlDataAdapter
Dim sqlquery = "SELECT * FROM client WHERE code ='"+ TxtCode.Text +"'"
                Dim comand As New MySqlCommand()
                comand.Connection = con
                comand.CommandText = sqlquery
                adap.SelectCommand = comand
                Dim data As MySqlDataReader
                data = comando2.ExecuteReader()
                leitor.Read()
                If (data(3).ToString) = code Then
                    MsgBox("already exists", MsgBoxStyle.Information)
                    TxtCode.ResetText()
                    TxtCode.Focus()
                Else
                    Console.WriteLine(insert("INSERT INTO client (name, tel, code) VALUES ('" & name & "', '" & tel & "')"))
                    con.Close()
                End If

最佳答案

您调用了leitor.Read()，但在代码中没有调用data.Read()
另外，由于您的查询是SELECT * FROM client WHERE code = '1234'，因此无需检查data(3) == code。如果存在记录，则数据.Read（）将为真。

If data.Read() Then
   MsgBox("already exists", MsgBoxStyle.Information)
   TxtCode.ResetText()
   TxtCode.Focus()
Else
   Console.WriteLine(insert("INSERT INTO client (name, tel, code) VALUES ('" & name & "', '" & tel & "')"))
   con.Close()
End If

使用参数而不是连接也是一个好的实践

  Dim sqlquery = "SELECT * FROM client WHERE code = @code"
  ...

  command.Parameters("@code", code); //this is safer

还有你的插页

 "INSERT INTO client (name, tel, code) VALUES (@name, @tel)"