SELECT 之后的筛选

改进 SQL Server 中的查询性能

Dean Smith

Founder, Atamai Analytics

SELECT 之后的处理顺序

1.  FROM

2.  ON

3.  JOIN

4.  WHERE

5.  GROUP BY

6.  HAVING


7.  SELECT


8.  DISTINCT

9.  ORDER BY

10. TOP

并非"全选"都好

SELECT *
FROM PlayerStats;

SELECT * 便于探索数据，但可能影响性能

PlayerName	Team	Position	...	...
Alex Abrines	OKC	SG	...	...
Quincy Acy	BRK	PF	...	...
Steven Adams	OKC	C	...	...
Bam Adebayo	MIA	C	...	...
...	...	...	...	...

只取所需更好

SELECT PlayerName, 
        Team, 
        TotalPoints
FROM PlayerStats;

仅选择需要的列

PlayerName	Team	TotalPoints
Alex Abrines	OKC	353
Quincy Acy	BRK	411
Steven Adams	OKC	1056
Bam Adebayo	MIA	477
...	...	...

JOIN 中的全选

SELECT * 
FROM Players p
  INNER JOIN PlayerStats ps
  ON ps.PlayerName = p.PlayerName;

在连接中使用 SELECT * 会返回连接列的重复项

PlayerName	Age	PlayerName	Team	...
Alex Abrines	24	Alex Abrines	OKC	...
Quincy Acy	27	Quincy Acy	BRK	...
Steven Adams	24	Steven Adams	OKC	...
Bam Adebayo	20	Bam Adebayo	MIA	...
...	...	...	...	...

JOIN 中你需要的一切

SELECT p.PlayerName, 
       ps.Team, 
       p.Country, 
       ps.Position
FROM Players p
  INNER JOIN PlayerStats ps
  ON ps.PlayerName = p.PlayerName;

明确要返回的列及其来源表

PlayerName	Country	Team	Position
Alex Abrines	Spain	OKC	SG
Quincy Acy	USA	BRK	PF
Steven Adams	New Zealand	OKC	C
Bam Adebayo	USA	MIA	C
...	...	...	...

取前几行（TOP）

SELECT TOP 5 PlayerName, 
        Team, 
        TotalPoints
FROM PlayerStats;

PlayerName	Team	TotalPoints
Alex Abrines	OKC	353
Quincy Acy	BRK	411
Steven Adams	OKC	1056
Bam Adebayo	MIA	477
Arron Afflalo	ORL	179

按百分比取前几行（TOP）

SELECT TOP 1 PERCENT PlayerName, 
        Team, 
        TotalPoints
FROM PlayerStats;

PlayerName	Team	TotalPoints
Alex Abrines	OKC	353
Quincy Acy	BRK	411
Steven Adams	OKC	1056
Bam Adebayo	MIA	477
Arron Afflalo	ORL	179
Cole Aldrich	MIN	12
LaMarcus Aldridge	SAS	1735

没有"顶部"或"底部"之分

SELECT TOP 5 PlayerName, 
        Team, 
        TotalPoints
FROM PlayerStats
ORDER BY TotalPoints DESC

PlayerName	Team	TotalPoints
LeBron James	CLE	2251
James Harden	HOU	2191
Anthony Davis	NOP	2110
Russell Westbrook	OKC	2028
Giannis Antetokounmpo	MIL	2014

其他行限制方式

行限制子句	排序子句	数据库
TOP	ORDER BY	Microsoft SQL Server
ROWNUM	ORDER BY	Oracle
LIMIT	ORDER BY	PostgreSQL

何时使用 ORDER BY

在查询中使用 ORDER BY：

用于探索数据
当确有理由让最终结果排序时

开始练习！

改进 SQL Server 中的查询性能