我如何使用T-SQL Group By [英] How do I use T-SQL Group By
本文介绍了我如何使用T-SQL Group By的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我知道我需要（尽管我不知道为什么）使用任何集合函数（如<$）的SQL查询末尾的 GROUP BY 子句c $ c> count ， sum ， avg 等：
I know I need to have (although I don't know why) a GROUP BY clause on the end of a SQL query that uses any aggregate functions like count, sum, avg, etc:
SELECT count(userID), userName
FROM users
GROUP BY userName
其他情况下 GROUP BY 是否有用，以及性能方面的影响是什么？
When else would GROUP BY be useful, and what are the performance ramifications?
推荐答案
要从每个具有5个窗口小部件的窗口小部件类别中检索窗口小部件的数量，您可以这样做：
To retrieve the number of widgets from each widget category that has more than 5 widgets, you could do this:
SELECT WidgetCategory, count(*)
FROM Widgets
GROUP BY WidgetCategory
HAVING count(*) > 5
having子句是人们经常忘记的东西，而是选择将所有数据检索到客户端并在那里迭代。
The "having" clause is something people often forget about, instead opting to retrieve all their data to the client and iterating through it there.
这篇关于我如何使用T-SQL Group By的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
查看全文
