January 2021
Intermediate
240 pages
5h 32m
Japanese
Content preview from 図解まるわかり データベースのしくみ
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140
データ同士の関係を考える
5-6
111
エンティティは他のエンティティと関係している場合が多く、エンティ
ティ同士の結びつきは、リレーションシップと呼ばれます（図5-10）。リ
レーショナル型データベースでは、複数の関連するテーブル同士を組み合
わせてデータを表現することになりますので、
。
リレーションシップには、以下の3 種類があります（図5-11）。
1対多
1 対多は1 つのデータに対して、複数のデータが関連している関係
を表します。例えば1 つの部署に対して複数の社員が存在している状
態ですね。また、SNSユーザーとその投稿の場合も、1 人のユーザー
に対して複数の投稿が存在しているため、1 対多の関係です。
多対多
多対多は、1 つのデータに対して複数のデータが関連しており、相
手側もこちらの複数のデータと結びつけられている関係です。授業と
学生の関係で例えると、1 つの授業に対して複数の学生が受講、逆に
1 人の学生は複数の授業を受講している、という具合ですね。
1対1
1 対 1 は、あるデータに対して対応する1 つのデータと結びつく関
係です。例えばサイトに登録したユーザーアカウントとメール受信設
定の情報は、1 人のユーザーごとに、対応する情報が結びつきます。
ただしテーブル設計時において
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.