mysql - 错误: Conversion from string "" to type 'Double' is not valid

Question

我需要选择最小自增数，然后选择id号，以供选择学生信息时引用。这是我得到的:

strSQL = "SELECT min(number) from main_queue "
cmd = New MySqlCommand(strSQL, CONNECTION)
dr = cmd.ExecuteReader
If dr.HasRows Then
    dr.Read()
    Dim min As Integer
    min = dr("min(number)")
    strSQL = "SELECT idno_ from main_queue where number='" + min + "'"
    cmd = New MySqlCommand(strSQL, CONNECTION)
    If dr.HasRows Then
        dr.Read()
        Dim idnum As Integer
        idnum = dr("idno_")
        strSQL = "SELECT * from student_records where number='" + idnum + "'"
        cmd = New MySqlCommand(strSQL, CONNECTION)
        If dr.HasRows Then
            dr.Read()
            mon_name.Text = dr("name_")
            mon_IDno.Text = dr("IDno")
        End If
    End If
End If

我收到“从字符串“”到类型“Double”的转换无效。”在我使用最小自动增量数字作为引用选择 id 数字的行中出现错误。

Answer 1

也许我没有正确理解您的数据库架构，但似乎您可以将其全部写入单个查询中

Dim strSQL = "SELECT * FROM student_records " & _
             "where idNO =  " & _
             "(SELECT idno_ from main_queue where number= " & _
             "(SELECT min(number) from main_queue))"
cmd = New MySqlCommand(strSQL, CONNECTION)
If dr.Read() Then
    mon_name.Text = dr("name_")
    mon_IDno.Text = dr("IDno")
End If