sample-by-percent_rank.sql

SELECT
  p.category,
  p.product_name,
  p.product_description
FROM (
  SELECT
    p.category,
    p.product_name,
    p.product_description,
   (percent_rank() OVER w1) * 100 AS percent_rnk
  FROM products p
  window w1 AS ( PARTITION BY p.category ORDER BY p.product_id)
)t1
WHERE t1.percent_rnk<50