OVER ที่นี่คืออะไร?

SELECT 
	-- Select the league name and average goals scored
	___ AS league,
    ___(___ + ___.away_goal) AS avg_goals,
    -- Rank each league over the average goals
    ___ ___(___ ___ AVG(___.home_goal + ___)) AS league_rank
FROM league AS l
LEFT JOIN match AS m 
ON l.id = m.country_id
WHERE m.season = '2011/2012'
GROUP BY l.name
-- Order the query by the rank you created
ORDER BY ___;