Premier League Predictor: Astro — Chapter 2, Exercise 1
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TASK
Explain why "which 20 teams are competing this season" is modeled as
a separate season_teams table rather than a column on teams itself,
and name the two later chapters that operate directly on season_teams
rows.

SOLUTION

"A team exists" and "a team is competing this season" are two
genuinely different facts with two different lifetimes. A team's own
identity is permanent — Leicester City doesn't stop existing the
season it gets relegated, and a relegated team may well be promoted
again in a future season. If "currently competing" were stored as a
plain boolean column on teams, that column would have to be
overwritten every single season for every team affected by promotion
or relegation, permanently destroying the record of which teams
competed in any earlier season the moment it changed.

season_teams instead records competing-in-a-season as its own real
fact: one row per (season, team) pairing. teams itself is never
touched by promotion or relegation at all — only season_teams rows
are added (for a promoted team) or simply not carried forward (for a
relegated one) when a new season starts. This preserves a genuine,
permanent historical record of exactly which 20 teams competed in
every past season, not just the current one.

The two chapters that operate directly on season_teams:

  - Chapter 3 (Admin: Managing the 20 Competing Teams Each Season) —
    its own CRUD tooling for the current season's roster is really
    just insert/delete against season_teams rows.
  - Chapter 9 (Promotion & Relegation) — removes the bottom three
    teams' own season_teams rows for the upcoming season and adds
    three new rows for the promoted teams, with teams itself
    completely unaffected either way.

WHY THIS WORKS AS AN ANSWER
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It identifies the real underlying distinction (permanent identity vs.
season-specific membership), explains concretely why a boolean column
would destroy historical data a join table preserves, and correctly
names both chapters the exercise asked for.