mysql - 在调用 Read() INSERT 之前访问字段的尝试无效

Question

我正在尝试使用这段代码来检查系统是否已经存在具有该值的字段

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

Answer 1

您在代码中调用了 leitor.Read() 但没有调用 data.Read()

此外，由于您的查询是 SELECT * FROM client WHERE code = '1234'，因此无需检查 data(3) == code。如果记录存在，则 data.Read() 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

使用参数而不是串联也是一个好习惯

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

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

和你的插入

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