Postgres :dapatkan jumlah baris min dan maks di banyak ke banyak tabel relasi

Bandingkan jumlah vendor dengan min dan max dari fungsi jendela dan jalankan hitungan bersyarat pada tingkat kueri luar:

SELECT vendor_id
     , count(min_bid OR NULL) AS min_bid_count
     , count(max_bid OR NULL) AS max_bid_count
FROM  (
   SELECT vendor_id
        , amount = min(amount) OVER w AS min_bid
        , amount = max(amount) OVER w AS max_bid
   FROM   rfq_vendor_mapping
   WINDOW w AS (PARTITION BY rfq_id)
   ) sub
GROUP  BY 1
ORDER  BY 1;

SQL Fiddle.