别名

改进 SQL Server 中的查询性能

Dean Smith

Founder, Atamai Analytics

什么是别名？

在查询中用于标识：
- 表
- 列
- 子查询
临时，仅在运行时生效
使查询更易读
有时是必需的

为什么用别名？

避免重复使用冗长的表/列名
易于识别连接的表及其列
标识新列
标识子查询
当连接表含同名列时避免歧义
重命名列

连接表——列名不明确

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

-----------------------------------------------
-- 错误，Pop2017 列同时存在于
Nations 和 Cities 表中

Ambiguous column name 'Pop2017'.

连接表——表名设别名

-- 表别名；Nations 为 n，Cities 为 c
SELECT n.CountryName, 
       n.Code2, 
       n.Capital, 
       c.Pop2017 -- 城市人口
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
...	...	...	...

重命名列

-- 列别名；
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
...	...	...	...

新列

-- 新列别名为 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
...	...	...

子查询

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 
              -- 子查询别名为 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
...	...	...

Passons à la pratique !

改进 SQL Server 中的查询性能