FROM のサブクエリ

SQL のデータ操作

Mona Khalil

Data Scientist, Greenhouse Software

FROM とサブクエリ

データを再構築し、変換
- SELECT する前にデータをロングからワイドに変換
- データの事前フィルタリング
集計値を再集計
- ホーム得点数の平均で、上位 3 チームはどこか？1. 各チームごとの AVG を計算
  1. AVG の値の上位 3 つを取得

FROM のサブクエリから...

SELECT
  t.team_long_name AS team,
  AVG(m.home_goal) AS home_avg
FROM match AS m
LEFT JOIN team AS t
ON m.hometeam_id = t.team_api_id
WHERE season = '2011/2012'
GROUP BY team;

| team                 | home_avg         |
|----------------------|------------------|
| 1. FC Köln           | 1.13725490196078 |
| 1. FC Nürnberg       | 1.27058823529412 |
| 1. FSV Mainz 05      | 1.43697478991597 |
| AC Ajaccio           | 1.12280701754386 |

...メインクエリへ！


FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team)

...メインクエリへ！


FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery

...メインクエリへ！

SELECT team, home_avg
FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery

...メインクエリへ！

SELECT team, home_avg
FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery
ORDER BY home_avg DESC
LIMIT 3;

| team           | home_avg |
|----------------|----------|
| FC Barcelona   | 3.8421   |
| Real Madrid CF | 3.6842   |
| PSV            | 3.3529   |

覚えておくべきポイント

1 つの FROM 句内に複数のサブクエリを作成可能
- エイリアスを忘れずに付ける
- 結合できる状態にする
FROM 句内でサブクエリをテーブルに結合可能
- 両方のテーブルに結合列が必要

練習しましょう！

SQL のデータ操作