mysql - How to get a percentage of total when the query has a GROUP BY?

Question

Say I have a non-normalized table with movie actor names and the movies they've been in. eg.

CREATE TABLE movies_actors (
  movies_actors_id INT,
  movie VARCHAR(255),
  actor VARCHAR(255),
  PRIMARY KEY (movies_actors_id)
);

I do a SELECT actor, COUNT(1) FROM movies_actors GROUP BY actor to find out how many movies the actor has been in. But I also want to find out what percentage of movies that actor has been in.

I guess I could do this:

SELECT
  actor,
  COUNT(1) AS total,
  COUNT(1) / (SELECT COUNT(1) FROM movies_actors) * 100 AS avg
FROM movies_actors
GROUP BY actor;

But that just seems... idk... yucky.

Any ideas?

Answer 1

For large sets, a JOIN may perform better than the subquery.

SELECT ma.actor
     , COUNT(1) AS total
     , COUNT(1) / t.cnt * 100 AS `percentage`
  FROM movies_actors ma
 CROSS
  JOIN (SELECT COUNT(1) AS cnt FROM movies_actors) t
 GROUP
    BY ma.actor
     , t.cnt  

For large sets, and when a large percentage of the rows are being returned, the JOIN operation can usually outperform a subquery. In your case, it's not a correlated subquery, so MySQL shouldn't have to execute that multiple times, so it may not make any difference.

Note to non-fans of COUNT(1)... we could replace any and all occurrences of COUNT(1) with COUNT(*) or IFNULL(SUM(1),0) to achieve equivalent result.