Aliasing

Meningkatkan Kinerja Kueri di SQL Server

Dean Smith

Founder, Atamai Analytics

Apa itu aliasing?

Digunakan dalam kueri untuk menandai:
- Tabel
- Kolom
- Subkueri
Sementara, hanya saat kueri dijalankan
Membuat kueri lebih mudah dibaca
Mungkin wajib

Mengapa memakai alias?

Hindari pengulangan nama tabel/kolom yang panjang
Mudah mengenali tabel join dan kolom terkait
Menandai kolom baru
Menandai subkueri
Hindari ambiguitas saat kolom bernama sama pada tabel join
Ganti nama kolom

Tabel join - nama kolom ambigu

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

-----------------------------------------------
-- ERROR, kolom Pop2017 ada di 
baik tabel Nations maupun Cities

Ambiguous column name 'Pop2017'.

Tabel join - alias nama tabel

-- Alias tabel; Nations sebagai n dan Cities sebagai c
SELECT n.CountryName, 
       n.Code2, 
       n.Capital, 
       c.Pop2017 -- Populasi kota
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
...	...	...	...

Ganti nama kolom

-- Alias kolom; 
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
...	...	...	...

Kolom baru

-- Kolom baru dialias sebagai MaxMagnitude
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
...	...	...

Subkueri

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 
              -- Subkueri dialias sebagai e
    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!

Meningkatkan Kinerja Kueri di SQL Server