SQL : find rows and sort according to number of matching columns?

Question

Let\'s imagine that we are having \"cars\" table with such a simple structure...

car_id INT
color ENUM(\'black\',\'white\',\'blue\')
weight ENUM(\'light\',\'         


        

Answer 1

Not overly efficient, but...

SELECT
  exact.car_id   AS e_car_id, exact.color   AS e_color, 
  exact.weight   AS e_weight, exact.type    AS e_type,
  related.car_id AS r_car_id, related.color AS r_color, 
  related.weight AS r_weight, related.type  AS r_type,
  CASE WHEN related.color = exact.color      THEN 1 ELSE 0 END
   + CASE WHEN related.weight = exact.weight THEN 1 ELSE 0 END
   + CASE WHEN related.type = exact.type     THEN 1 ELSE 0 END
  AS rank
FROM
  cars AS exact
  INNER JOIN cars AS related ON (
    related.car_id <> exact.car_id 
    AND CASE WHEN related.color = exact.color   THEN 1 ELSE 0 END
      + CASE WHEN related.weight = exact.weight THEN 1 ELSE 0 END
      + CASE WHEN related.type = exact.type     THEN 1 ELSE 0 END
    >= 1
  )
WHERE
  exact.car_id = 1 /* black, heavy, limo */
ORDER BY
  rank DESC

This would not run very fast on large data sets, since neither the JOIN nor the ORDER BY can make use of an index. Very probably a more optimal version exists.

Output on my test setup looks like this:

e_car_id  e_color  e_weight  e_type  r_car_id  r_color  r_weight  r_type  rank
1         black    heavy     limo    7         black    heavy     limo    3
1         black    heavy     limo    2         black    light     limo    2
1         black    heavy     limo    3         black    heavy     van     2
1         black    heavy     limo    4         black    medium    van     1
1         black    heavy     limo    5         blue     light     limo    1