SELECT hr,
       bmex / gdax AS ratio
FROM
  (SELECT hr,
          max(gdax) "gdax", -- max(..) necessary to grab the non-NULL row from two returned
          max(bmex) "bmex"  -- rows for each hr
   FROM
     (SELECT hr,
             (CASE
                  WHEN exch=3 THEN wt_avg
              END) "gdax",
             (CASE
                  WHEN exch=6 THEN wt_avg
              END) "bmex"
      FROM
        (SELECT hr,
                exch,
                w_sum / sum_w AS wt_avg
         FROM
           (SELECT date_trunc('hour', "time") AS hr,
                   exch,
                   sum(price * amount) AS w_sum,
                   sum(amount) AS sum_w
            FROM trades
            WHERE base=1             -- btc=1 usd=100
              AND quote=100
              AND (exch=3 OR exch=6) -- gdax=3, bmex=6
            GROUP BY 1, 2
            ORDER BY 1, 2) a) b) c
   GROUP BY hr) d;