Error: Conversion from string “” to type 'Double' is not valid

Question

i need to select the minimum auto-increment number, then select the id number for reference in selecting the student's info. here's what I've got:

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

I'm getting a "Conversion from string "" to type 'Double' is not valid." error at line where i select the id number using the minimum auto-increment number as the reference.

Answer 1

Perhaps I don't understand correctly your database schema, but it seems that you could write it all in a single query

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

managed to make it work! here's my code

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