Find sum of SOME items from two mysql tables

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VB

MySQL

Hi!

So, I have two tables, exam1 and exam2 which looks like these:

table:exam1

sid       physics     chemistry       biology
3          80             60            40

   table:exam2
sid        physics    chemistry       biology
3            40          50              70

[b]Problem:[/b]
I want to find the sum of items from both tables (exam1 and exam2), like this

table: total
sid         physics         chemistry       biology
3            120               110            110

[b]BUT:[/b]
I wont always have to find total of all courses (physics, chemistry, biology), sometimes I may want to find sum of only two, one of all three subjects, so this is really a variable.

Since I program from VB.NET, the list of courses to be totaled can be specified by a user at run time and is kept in a variable say, "courses", so sometimes courses= (physics,chemistry,biology) sometimes courses=(physics,chemistry) and so on.

How can I then find a total of courses that the user selects? (still total of courses in exam1+exam2 tables)

Thanks

What I have tried:

<pre lang="vb">

Dim courses As String = "physics,chemistry"

        Dim cmd As New MySqlCommand
        cmd.CommandText = "Select A.sid, (A." & LCase((courses)) & "+B." & LCase((courses)) &
                            ") As " & courses & " From exam1 A Inner Join exam2 B On A.sid=B.sid Order By A.sid"

        MsgBox("STATEMENT = " & cmd.CommandText)
        cmd.Connection = con
        Dim rd As MySqlDataReader = cmd.ExecuteReader()


        If rd.HasRows() Then
            Dim x As String
            Do While rd.Read()
                x &= String.Format("{0}  {1}", rd("sid"), rd(ComboBox2.SelectedItem)) & vbCrLf
            Loop
            MessageBox.Show(x)
        End If
        rd.Close()
        cmd.Dispose()
        con.Dispose()