我如何使用 T-SQL Group By

Question

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

When else would GROUP BY be useful, and what are the performance ramifications?

Answer 1

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

The "having" clause is something people often forget about, instead opting to retrieve all their data to the client and iterating through it there.

Answer 2

GROUP BY is similar to DISTINCT in that it groups multiple records into one.

This example, borrowed from http://www.devguru.com/technologies/t-sql/7080.asp , lists distinct products in the Products table.

SELECT Product FROM Products GROUP BY Product

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

The advantage of GROUP BY over DISTINCT, is that it can give you granular control when used with a HAVING clause.

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

Answer 3

Group By forces the entire set to be populated before records are returned (since it is an implicit sort).

For that reason (and many others), never use a Group By in a subquery.

Answer 4

Counting the number of times tags are used might be a google example:

SELECT TagName, Count(*)
AS TimesUsed
FROM Tags
GROUP BY TagName ORDER TimesUsed

If you simply want a distinct value of tags, I would prefer to use the DISTINCT statement.

SELECT DISTINCT TagName
FROM Tags
ORDER BY TagName ASC

Answer 5

GROUP BY also helps when you want to generate a report that will average or sum a bunch of data. You can GROUP By the Department ID and the SUM all the sales revenue or AVG the count of sales for each month.