August 2024
Intermediate to advanced
392 pages
10h 4m
Japanese
Content preview from 達人に学ぶDB設計徹底指南書 第2版
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テーブルの列も同じで、配列ほど柔軟に要素数を変化させることができません。
たとえば、四人の子を持つ社員が入社してきたので、この扶養者テーブルに「子 4」
という列を追加しようと思った場合、変更対象はこのテーブルだけでなく、SQL や
データを受け取るアプリケーション側にも及びます。もちろん、ある程度そういっ
た変更を見越して動的にアプリケーションを作り込んでおくことも可能なのです
が、なかなか大変なことです。
列持ちテーブルのもう一つの欠点は、子がいない、または子の数の少ない社員
について、子の列にNULLを使わなくてはならないことです。NULLというのはリ
レーショナルデータベースにおいて非常に厄介な性質を持っていて、NULL が演算
に含まれているとSQL文の結果を混乱させる原因になります
。
このように、列持ちテーブルは、利点と欠点のバランスが拮抗しているため、使
いどころによっては有用です。逆に言うと、特殊な状況でない限り、原則として列
持ちテーブルは使うべきではありません。基本的には「行持ち」のテーブル構成を
採用するべきです。行持ちテーブルは、列持ちテーブルに比べて、欠点の少ない定
石です。
000A 1 達夫
000A 2 信二
001F 1 敦
001F 2 陽子
001F 3 清美
...
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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.