MySQL - Calculate percentage of rows/records matching predicate using IF() and AVG

percentage-mysql-avg.sql

-- Tip: You can use IF() and AVG() together to get a percentage value
--      without doing subquerys
-- -
-- Basic usage:
-- SELECT AVG(IF( predicate ,100,0)) as percentage FROM table
-- Example:
-- -
SELECT 
  AVG(IF(f.mid,100,0)) as `percentage filed`
FROM
  Nominations n, Movies m LEFT JOIN Filings f ON f.mid=m.id
WHERE
  n.mid=m.id AND  n.yid=3
-- -
-- Read: 
-- "What portion of the Nominations with yid=3 have a Filing linked to their movie?"
-- -