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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86
テーブルを結合して
データを取得する
3-20
2-3で解説しましたが、2 つ以上のテーブルを結合して まとめてデー
タを取得することをテーブル結合と呼びます。ここでは実際に、コマンド
を使ってテーブル結合するにあたっての必要な知識を解説していきます。
コマンドを使ってのテーブル結合は、結合元となるテーブルに、結合先
となるテーブルをくっつけるイメージです。その際、結合元と結合先それ
ぞれのテーブル名と、2つのテーブル同士で共通のキーとなる値を格納し
ておくカラムの名前が必要です（図3-44）。そしてコマンドの中で「」
を用いてこれらの要素を指定することで、テーブル結合が実現できます。
例えば図書館のデータベースを例に考えてみましょう。図3-45 のよう
な「貸し出し日」と「本の ID」が保存されている貸し出し履歴テーブル
があったとします。このテーブルだけだと本のタイトルやジャンルはわか
りません。そのため本の情報テーブルと結合してまとめてデータを取得し
たくなりました。その場合は、結合元となる貸し出し履歴テーブルに、結
合先として本の情報テーブルをくっつけて取得します。このとき結合に必
要な要素として以下が挙げられます。
結合元のテーブル名:貸し出し履歴／結合元のカラム名 :本のID
結合先のテーブル名:本の情報／結合先のカラム名 :ID
テーブルの結合の種類には、内部結合と外部結合があります。
内部結合は、テーブル同士でキーとなるカラムの値が一致するデータの ...
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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.