SQL - 条件ロジック(結果セットの変換)

初めてのSQL
Alan Beaulieu (著)

開発環境

• OS: OS X Lion - Apple
• データベース言語: SQL
• リレーショナルデータベース: MySQL

『初めてのSQL』(Alan Beaulieu 著、株式会社クイープ 翻訳、オライリー・ジャパン、2006年、ISBN4-87311-281-8) の11章(条件ロジック),　11.4(練習問題)11-2を解いてみる。

11-2.

SQL文(TextWrangler)

select
sum(case
when open_branch_id = 1 then 1 else 0 end) branch_1,
sum(case
when open_branch_id = 2 then 1 else 0 end) branch_2,
sum(case
when open_branch_id = 3 then 1 else 0 end) branch_3,
sum(case
when open_branch_id = 4 then 1 else 0 end) branch_4
from account;

入出力結果(Terminal)

$ mysql -u lrngsql -p bank
Enter password: 
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 18
Server version: 5.5.24 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select
    -> sum(case
    -> when open_branch_id = 1 then 1 else 0 end) branch_1,
    -> sum(case
    -> when open_branch_id = 2 then 1 else 0 end) branch_2,
    -> sum(case
    -> when open_branch_id = 3 then 1 else 0 end) branch_3,
    -> sum(case
    -> when open_branch_id = 4 then 1 else 0 end) branch_4
    -> from account;
+----------+----------+----------+----------+
| branch_1 | branch_2 | branch_3 | branch_4 |
+----------+----------+----------+----------+
|        8 |        7 |        3 |        6 |
+----------+----------+----------+----------+
1 row in set (0.22 sec)

mysql> quit;
Bye
$