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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124
ひとまとまりの実行した処理を
なかったことにする
4-16
トランザクション内の処理で問題が発生したときに、処理を取り消して
トランザクション開始時点の状態まで戻すことをロールバックと呼びま
す。データベース上で実行される処理は常に正常に行われるとは限りませ
ん。プログラムにおけるバグや、ネットワーク障害によってデータベース
に接続できなくなるなど、さまざまな想定外の問題が起こります。そのよ
うなときにトランザクション内の処理を途中で中断してしまうと、データ
の整合性がとれなくなってしまうことがあります。これを防ぐために、ロ
ールバックを用いて
します（図4-31）。
銀行口座の例で考えてみましょう。A口座から B口座に 10 万円を送金す
るときは、正常な場合、データベース上ではトランザクションを用いて
「A口座の預金をマイナス10 万円にする」「 B口座の預金をプラス10 万円
にする」といった2 つの処理を実行し、最後にコミットという流れになり
ます。
しかし、A口座の値を更新した直後に問題が発生し、処理が継続できな
くなってしまった場合、このままコミットを行ってしまうと、B口座の更
新がまだ行われていないのでデータの整合性が合わなくなってしまいま
す。このようなときにロールバックを行って、トランザクション開始時点
の状態まで巻き戻します。結果として
...
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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.