July 2021
Intermediate to advanced
376 pages
6h 34m
Japanese
Content preview from 初めてのSQL 第3版
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付録AサンプルデータベースのER図
図A-1は、本書で使っているサンプルデータベースのER（Entity-Relationship）図です。その名前からもわかるように、ER図はデータベースのエンティティ（テーブル）と、テーブル間の外部キー関係を表します。次に、ER図の表記を理解するためのヒントをまとめておきます。
- 四角形はそれぞれテーブルを表している。テーブルの名前は四角形の左上に表示される。最初に主キー列が列挙され、続いてキーではない列が列挙される。
- テーブルどうしを結ぶ線は外部キー関係を表している。行の終わりに付いているマークは許可される量を表しており、0（○）、1（|）、複数（<）のいずれかになる。たとえば、
customerテーブルとrentalテーブルの関係を見てみると、レンタルがいずれか1人の顧客に紐付けられることと、顧客が0、1、または複数のレンタルに紐付けられることがわかる。
ERモデルの詳細については、Wikipedia†1などを参照してください。
[†1] https://en.wikipedia.org/wiki/Entity%E2%80%93relationship_modelhttps://ja.wikipedia.org/wiki/実体関連モデル
図A-1 ER図
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