一致するrowだけを得るのがINNER JOIN。
左のテーブルrowをそのまま残すのがLEFT JOIN。
一致するものはくっつけ、しないものもblank表示するのがFULL JOIN。
2つのテーブルの行のすべての組み合わせを表示するのがCROSS JOIN。
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JOINの構文。ONが共通データを持つ前提でテーブルを合体。
FROM table_a
JOIN table_b
ON table_a.id = table_b.id
例。
SELECT teams.conference AS conference,
AVG(players.weight) AS average_weight
FROM benn.college_football_players players
JOIN benn.college_football_teams teams
ON teams.school_name = players.school_name
GROUP BY teams.conference
ORDER BY AVG(players.weight) DESC