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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156
カラムに割り当てる設定を決める
5-14
データを保存するのに必要なカラムを決めたら、今度はそれぞれのカラ
ムに割り当てるデータ型や制約、属性を決める必要があります。
まずデータ型については、カラムごとに保存する値のフォーマットに応
じて、、、などを割り当てる必要があります。
また、制約や属性については、初期値を入れるか、データが空の状態を
許可しないようにするか、他のレコードと同じ値を入れられないようにす
るか、自動で連番を格納するか、主キーや外部キーにするか、などの観点
からカラムに割り当てる設定を決めていきます（データ型や制約の種類に
ついては第4 章参照）。
図5-25 は、カラムの設定を割り当てた例です。
それぞれのテーブルにある講義ID、教員ID、学生IDは主キーとし、自
動的に連番を割り当てることによって、レコードを一意に絞り込めるよう
にするためのカラムとしてあります。
そして講義名や教員名、学生名などのカラムについては、空欄で保存さ
れないようにするため、空の状態を許可しない設定を追加してあります。
他のテーブルの値に紐付くカラムについては外部キーにしておくこと
で、参照元のテーブルに存在しない値を保存できないようにしてありま
す。
ちなみに教員連絡先は数字の羅列が保存されることを考慮して、数値型
としてあります。ハイフン（-）も入れて保存させたいなど、場合によっ
ては文字列型にすることもあるでしょう。
カラムのデータ型、
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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.