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SQL Server 中级

Ginger Grant

Instructor

课程概览

第 1 章：数据汇总
第 2 章：日期与数学函数
第 3 章：用 T-SQL 处理数据
第 4 章：窗口函数

用聚合探索数据

查看各列的汇总值是分析数据的常见第一步
若数据在数据库中，最快的聚合方式是用 SQL

EconomicIndicators 数据探索

SELECT Country, Year, InternetUse, GDP,
       ExportGoodsPercent, CellPhonesper100 
FROM EconomicIndicators

+-----------+-----+------------+------------+------------------+-----------------+
|Country    |Year |InternetUse | GDP        |ExportGoodsPercent|CellPhonesper100 | 
+-----------+-----+------------+------------+------------------+-----------------+
|Swaziland  |2011 |20.43165813 |7335004354  |56.30476059       |63.7015615       |
|Sweden     |2011 |90.88204559 |394271163688|49.93022195       |118.5711258      |
|Switzerland|2011 |82.98773087 |395111518596|51.20242546       |130.0623629      |
...
+-----------+-----+------------+------------+------------------+-----------------+

常用汇总统计

MIN()：列的最小值
MAX()：列的最大值
AVG()：列的平均值

T-SQL 中的常用汇总统计

此 T-SQL 查询返回 InternetUse 列的聚合值

SELECT AVG(InternetUse) AS MeanInternetUse, 
MIN(InternetUse) AS MINInternet, 
MAX(InternetUse) AS MAXInternet 
FROM EconomicIndicators


+-------------------+------------+-------------+
|MeanInternetUse    |MINInternet |  MAXInternet|
|-------------------+------------+-------------|
|   18.9854496196171|          0 |  375.5970064|
+-------------------+------------+-------------+

用 WHERE 筛选汇总数据

此 T-SQL 查询使用 WHERE 子句筛选聚合值
注意文本值为

SELECT AVG(InternetUse) AS MeanInternetUse, 
MIN(InternetUse) AS MINInternet, 
MAX(InternetUse) AS MAXInternet 
FROM EconomicIndicators
WHERE Country = 'Solomon Islands'

+-------------------+------------+-------------+
|MeanInternetUse    |MINInternet |  MAXInternet|
|-------------------+------------+-------------|
|            1.79621|          0 |         6.00|
+-------------------+------------+-------------+

用 GROUP BY 分组小计

SELECT Country, AVG(InternetUse) AS MeanInternetUse, 
MIN(InternetUse) AS MINInternet, 
MAX(InternetUse) AS MAXInternet 
FROM EconomicIndicators
GROUP BY Country

+--------------------+------------------+------------+-------------+
|  Country           |MeanInternetUse   |MINInternet |  MAXInternet|
|--------------------+------------------+------------+-------------|
|Solomon Islands     |           1.79621|           0|         6.00|
|Hong Kong           |          245.1067|           0|       375.00|
|Liechtenstein       |           63.8821|     36.5152|        85.00|
...
+--------------------+------------------+------------+-------------+

HAVING 是聚合的 WHERE

不能将 WHERE 与 GROUP BY 一起使用，否则会报错

-- 这会报错
...
GROUP BY
WHERE Max(InternetUse) > 100

应改用 HAVING

-- 带 GROUP BY 的正确筛选方式
...
GROUP BY
HAVING Max(InternetUse) > 100

HAVING 是聚合的 WHERE

SELECT Country, AVG(InternetUse) AS MeanInternetUse, 
MIN(GDP) AS SmallestGDP,
MAX(InternetUse) AS MAXInternetUse
FROM EconomicIndicators
GROUP BY Country
HAVING MAX(InternetUse) > 100

+--------------+-------------------+-------------+---------------+
|Country       |MeanInternetUse    |SmallestGDP  | MAXInternetUse|
|--------------+-------------------+-------------+---------------|
|Macedonia     |   71.3060150792857| -0.465059948|    110.5679538|
|Hong Kong     |   245.106718614286|            0|    375.5970064|
|Congo         |   60.8972476010714| -9.492757847|    104.6455529|
...
+--------------+-------------------+-------------+---------------+

查看 Incidents 表中的 UFO 数据

练习将探索来自 Mutual UFO Network 的数据
全球的不明飞行物目击记录在 Incidents 表中

Passons à la pratique !

SQL Server 中级