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SQL: JOINで2つのテーブルをまとめる。

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
Ratio13
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