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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5
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SQL、特に複数のテーブルを結合するような複雑なSQLを考えるときのコツ
は、「FROM句から考える」ことです。これはつまり、「このSQL を書くために必
要なテーブルは何だろう」と考えることです。SQL の初心者は、SQL 文の先頭、
つまりSELECT 句から考えがちです。しかしSELECT句というのは、最後に決まる
要素であるため、実はFROM 句→WHERE句→GROUP BY句……という順序で考
えたほうが簡単なのです。
そのやり方に従って考えてみましょう。今回の問題を解くのに必要なテーブルは
何でしょうか。まず、商品分類名が必要とされることから、「商品分類」テーブル
が必要です。次に、商品の個数を数える必要があることから、「商品」テーブルも
必要です。この二者が FROM 句で必要になるテーブルです。
その両者を「商品分類コード」によって結合した後に、商品分類ごとのレコード
344
...
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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.