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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94
保存できるデータの種類を
指定する
4-1
3-5ではテーブルを作成するときにカラム（列）の名前と、そのデー
タ型を指定するという話をしました。テーブルのそれぞれのカラムには、
必ずデータ型を決めておく必要があります（図 4-1）。データ型を指定す
ることで、ことができた
り、値をどのように扱うか決めておいたりすることができます。
データ型にはいくつか種類があり、大きく分類すると、
数値を扱う型
文字列を扱う型
日付、時間を扱う型
といったものがあります。具体的な型の種類はこの後紹介していきます。
例えば金額を保存するカラムに整数型を指定するとします。すると、そ
のカラムには必ず整数しか保存することができなくなり、小数や文字が代
入されることがなくなります。
また、整数型にしておくことで保存してある値を数値として取得するこ
とができるので、計算に用いることもできます。例えば3-17 で紹介した
SUM関数を使えば、売上の合計金額を取得することができます。さらに
3-9で紹介したように値が 300以上のレコードを検索することも可能にな
ります。これは文字列の型だとできません。型によって値の扱い方も変わ
ります（図4-2）。
このように、保存できる値が制限され、取得するときの扱い方が変わる
ので、しておくことが重要です。
データ型の指定
データ型の役割
95
4
Point
各カラム（
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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.