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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104
保存できるデータに制限をつける
4-6
テーブルのカラムに対して制約をつけて格納できるデータに制限をかけ
たり、属性をつけて値をある規則で整えて格納することができます（図
4-11）。例えばそのカラムには必ず何か値を代入しないといけないNOT
NULL制約や、連番を自動的に格納するAUTO̲INCREMENT属性などがあ
ります（図4-12）。データの挿入や変更時にカラムの制約に引っかかった
場合はエラーとなり、処理が行われません。そのため
ことできます。また、
メリットもあります。
ここでは代表的な制約や属性の例を紹介します。
NOTNULL
NULL（4-8参照）を保存できない制約です。この制約がついてい
るカラムには、必ず何らかの値が入っている必要があります。
UNIQUE
カラムの値を重複させない制約です。この制約がついているカラム
には、他のレコードの値と同じ値は格納できません。
DEFAULT
カラムの値にデフォルト値を設定する制約です。この制約がついて
いるカラムに値を指定しなかった場合は、あらかじめ指定したデフォ
ルト値が格納されます。
AUTO̲INCREMENT
カラムに自動で連番を入れる属性です。この属性がついているカラ
ムには、自動的に連続した数字が入ります。
ルールに一致しないデー
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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.
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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.