How do I use 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,

Answer 1

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