錯誤：從字符串“”到類型“ Double”的轉換無效

Question

我需要選擇最小的自動遞增數字，然后選擇ID號以供選擇學生信息時參考。 這是我得到的：

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“從字符串“”到類型“ Double”的轉換無效。” 在我使用最小自動遞增數字作為參考來選擇ID號的行上出現錯誤。

Answer 1

也許我不能正確理解您的數據庫架構，但是似乎您可以在一個查詢中將其全部寫入

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

Answer 2

設法使它起作用！ 這是我的代碼

SELECT * from student_records where IDno in (select idno_ from main_queue where number in (SELECT min(number) as min_number from main_queue))