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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112
他の行と同じ値を
入れられないようにする
4-10
UNIQUE制約を使うと、カラムに他のレコードと重複した値を格納で
きないようにすることができます。もしUNIQUE制約が設定されたカラム
に対して重複した値を格納しようとした場合は、エラーとなって登録でき
ません（図4-19）。
例えば商品コードやユーザーIDのような、
といった用途が考えられるでしょう。もし別
の商品に同じ商品コードがついていると、識別できずに困ってしまいま
す。あらかじめUNIQUE制約を設定しておくことによってそのような重複
を防ぐことが可能になります。
ちなみにNULLは値がないということを表していますが（4-8 参照）、
こちらはUNIQUE制約が適用されません。例外として複数のレコードに格
納することができます。UNIQUE制約は値が存在するレコードにのみ適用
されるということになります。
MySQLの場合は、図4-20 のようにテーブル作成時に「UNIQUE」をカ
ラム名の後ろにつけることで、重複した値を格納できないように設定する
ことができます。今回の例では「id」カラムと「name」カラムを設けた
「users」テーブルを作成し、「id」カラムにUNIQUE制約を設定していま
す。
このテーブルに「id」カラムを「1」、「name」カラムを「山田」にセッ
トしてレコードを追加（3-6参照）してみます。続けて今度は「id」カラ
ムを「1」、「name ...
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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.