How Do I Retrieve Data From Multiple Tables

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C#

.NET

Hi, please i am implementing a result system whereby the names of students from their respective classes are retrieved based on a unique ID and their results are entered in the respective subject fields. i wanted to implement this with a formview, whereby the user can enter the student Id and class and the student name is retrieved along with the subject fields, then the subjects can be entered. i tried this;

conn.Open();
        String sql = "select * from" + classDropDown.Text + "where id='" + idtbox.Text + "'";
        using (SqlDataAdapter adapter = new SqlDataAdapter(sql, conn))
        {

            DataTable table = new DataTable();
            adapter.Fill(table);
            FormView1.DataSource = table;
            FormView1.DataBind();
        }
    }

since each class has a different table, but this is not working. Please suggest to me what other method to use or how i can implement this using formview control.
Thanks for taking time out to read this.