Unions and intersections with sets

UNION is used to combine output from multiple queries. By default it removes duplicate results. Parentheses are not required but could be used to remove ambiguity, such as ORDER BY and LIMIT at the end of the following query.

(
  SELECT *
  FROM products
  ORDER BY price DESC
  LIMIT 4
)
UNION
(
  SELECT *
  FROM products
  ORDER BY price / weight DESC
  LIMIT 4
);

To show duplicate results, use UNION ALL.

(
  SELECT *
  FROM products
  ORDER BY price DESC
  LIMIT 4
)
UNION ALL
(
  SELECT *
  FROM products
  ORDER BY price / weight DESC
  LIMIT 4
);