fix distinct_on for mysql with a single column

refs CNVS-7032

Change-Id: Iefcdeeed62d718d52ce120cfa689b31d308e9004
Reviewed-on: https://gerrit.instructure.com/29057
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>