別名（エイリアス）

SQL Serverでクエリ性能を改善する

Dean Smith

Founder, Atamai Analytics

別名とは？

次で使用:
- テーブル
- 列
- サブクエリ
一時的。実行時のみ有効
可読性を向上
必要な場合がある

別名を使う理由

長いテーブル名・列名の反復を回避
結合したテーブルと対応列を識別しやすく
新しい列を識別
サブクエリを識別
同名列によるあいまいさを回避
列名を変更

結合時の同名列のあいまいさ

SELECT CountryName, 
       Code2, 
       Capital, 
       Pop2017
FROM Nations
INNER JOIN Cities
  ON Capital = CityName;

-----------------------------------------------
-- エラー: Pop2017 列は
Nations と Cities の両方に存在

Ambiguous column name 'Pop2017'.

結合時のテーブル別名

-- テーブルに別名: Nations を n、Cities を c
SELECT n.CountryName, 
       n.Code2, 
       n.Capital, 
       c.Pop2017 -- 都市の人口
FROM Nations AS n
INNER JOIN Cities AS c
  ON n.Capital = c.CityName;

CountryName	Code2	Capital	Pop2017
United Kingdom	GB	London	346774
Canada	CA	Ottawa	874433
France	FR	Paris	10437
Reunion	RE	Saint-Denis	1067
...	...	...	...

列名の変更

-- 列に別名を付与
SELECT n.CountryName AS Country, 
       n.Code2 AS CountryCode, 
       n.Capital, 
       c.Pop2017 AS Population
FROM Nations AS n
INNER JOIN Cities AS c
  ON n.Capital = c.CityName;

Country	CountryCode	Capital	Population
United Kingdom	GB	London	346774
Canada	CA	Ottawa	874433
France	FR	Paris	10437
Reunion	RE	Saint-Denis	1067
...	...	...	...

新しい列

-- 新しい列に MaxMagnitude の別名
SELECT Country, 
       NearestPop AS City,
       MAX(Magnitude) AS MaxMagnitude
FROM Earthquakes 
GROUP BY Country, NearestPop;

Country	City	MaxMagnitude
PE	Acar	7.1
US	Aguadilla	7.7
MX	Aguililla	7.2
PW	Airai	7.8
PG	Aitape	7.6
...	...	...

サブクエリ

SELECT n.CountryName AS Country, 
       n.Capital, 
       e.MaxMagnitude
FROM Nations n
INNER JOIN
        (SELECT Country, NearestPop AS City
                ,MAX(Magnitude) AS MaxMagnitude
        FROM Earthquakes 
        GROUP BY Country, NearestPop) e 
              -- サブクエリに e の別名
    ON n.Code2 = e.Country AND n.Capital = e.City;

Country	Capital	MaxMagnitude
Fiji	Suva	7.9
Guam	Hagatna	7.8
Peru	Lima	7.6
Turkmenistan	Ashgabat	7.3
...	...	...

Ayo berlatih!

SQL Serverでクエリ性能を改善する