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
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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.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.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.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.
76
データの数を取得する
3-15
ユーザー情報が保存されているテーブルから登録されているユーザーが
何人いるか、図書館の本のテーブルから所蔵されている本の総数、スケジ
ュール情報が登録されているテーブルからタスク数を確認する場合、
こ
とができます。
テーブルに保存されているレコード数を取得するには、COUNT 関数
を使います。図3-34 は「users」テーブルからレコードの数を取得する例
です。「SELECT」の後ろに「COUNT(*)」を入れると、該当するレコード
数が取得でき、今回の例では「4」という結果が返ってきます。
レコード数をカウントするときに、3-8 で紹介した「WHERE」と組み
合わせることもできます。
図3-35 の例では、「users」テーブルの「age」カラムの値が「30」以上
の条件に当てはまるレコード数を取得しています。この他にも男性の人数
や女性の人数をカウントしたり、本の情報を保存しているテーブルから発
売日を検索条件に使って、今日新しく発売された本の数を取得したりと、
さまざまな使い方ができます。
値がないフィールドはNULLで表現されます（4-8参照）が、NULL の
データを除外してカウントすることもできます。
「SELECT」の後ろを「COUNT(age)」と指定すると、「age」カラムの値
がNULL のデータを除外してカウントすることができます。
レコード数のカウン ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access
More than 5,000 organizations count on O’Reilly
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.