我如何使用T-SQL Group By

Question

我知道我需要(虽然我不知道为什么),一个GROUP BY在任何使用聚合函数类似SQL查询的结尾条款count,sum,avg,等:

SELECT count(userID), userName
FROM users
GROUP BY userName

什么时候GROUP BY会有用,什么是性能分支？

Answer 1

要从具有5个以上小部件的每个窗口小部件类别中检索窗口小部件的数量,您可以执行以下操作:

SELECT WidgetCategory, count(*)
FROM Widgets
GROUP BY WidgetCategory
HAVING count(*) > 5

"having"子句是人们经常忘记的,而是选择将所有数据检索到客户端并在那里迭代.

Answer 2

GROUP BY类似于DISTINCT,因为它将多个记录分组为一个.

此示例借用于http://www.devguru.com/technologies/t-sql/7080.asp,列出了Products表中的不同产品.

SELECT Product FROM Products GROUP BY Product

Product
-------------
Desktop
Laptop
Mouse
Network Card
Hard Drive
Software
Book
Accessory

GROUP BY优于DISTINCT的优点是,当与HAVING子句一起使用时,它可以为您提供精细控制.

SELECT Product, count(Product) as ProdCnt
FROM Products
GROUP BY Product
HAVING count(Product) > 2

Product      ProdCnt
--------------------
Desktop          10
Laptop            5
Mouse             3
Network Card      9
Software          6