I suppose if you need a style guide for SQL, then you're likely in the business of building databases to be accessed by multiple applications. However, be aware that the majority of applications are initially developed using a database as a data store, alongside opinionated ORM systems that will prefer a specific naming convention (such as plural forms).

Speaking from experience, you can't even expect native speakers to know all the collective forms. Worse, in some cases the collective form may cause confusion if the collective is the singular of a different table (players in a team, teams in a league, doing a fantasy football league app).

Using the plural, your schema would be player -> team -> league. Using your system, it would be team -> league -> sport. Good luck explaining that to a new hire.