Join vs. sub-query

Question

I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query, and I hate it; I don\'t know why.

Answer 1

Run on a very large database from an old Mambo CMS:

SELECT id, alias
FROM
  mos_categories
WHERE
  id IN (
    SELECT
      DISTINCT catid
    FROM mos_content
  );

0 seconds

SELECT
  DISTINCT mos_content.catid,
  mos_categories.alias
FROM
  mos_content, mos_categories
WHERE
  mos_content.catid = mos_categories.id;

~3 seconds

An EXPLAIN shows that they examine the exact same number of rows, but one takes 3 seconds and one is near instant. Moral of the story? If performance is important (when isn't it?), try it multiple ways and see which one is fastest.

And...

SELECT
  DISTINCT mos_categories.id,
  mos_categories.alias
FROM
  mos_content, mos_categories
WHERE
  mos_content.catid = mos_categories.id;

0 seconds

Again, same results, same number of rows examined. My guess is that DISTINCT mos_content.catid takes far longer to figure out than DISTINCT mos_categories.id does.