Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 3: The Foundation Statements of T-SQL
Aggregating Data Using the GROUP BY Clause
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With ORDER BY, we have kind of taken things out of order compared with how the SELECT statement
reads at the top of the chapter. Let’s review the overall statement structure:
SELECT [TOP () [PERCENT] [WITH TIES]]
[FROM ]
[WHERE ]
[GROUP BY ]
[HAVING ]
[ORDER BY ]
[[FOR XML {RAW|AUTO|EXPLICIT|PATH [()]}[, XMLDATA][, ELEMENTS][, BINARY
base 64]]
[OPTION (, [, ...n])]
Why, if ORDER BY comes last, did we look at it before the GROUP BY? There are two reasons:
❑ ORDER BY is used far more often than GROUP BY, so I want you to have more practice with it.
❑ I want to make sure that you understand that you can mix and match all of the clauses after the
FROM clause, as long as you keep them in the order that SQL Server expects them (as defined in
the syntax definition).
The GROUP BY clause is used to aggregate information. Let’s look at a simple query without a GROUP BY.
Let’s say that we want to know how many parts were ordered in a given set of orders:
SELECT SalesOrderID, OrderQty
FROM Sales.SalesOrderDetail
WHERE SalesOrderID IN (43660, 43670, 43672);
This yields a result set of:
SalesOrderID OrderQty
------------ --------
43660 1
43660 1
43670 1
43670 2
43670 2
43670 1
43672 6
43672 2
43672 1
(9 row(s) affected)
Even though we’ve only asked for three orders, we’re seeing each individual line of detail from the
orders. We can either get out our adding machine, or we can make use of the GROUP BY clause with an
aggregator. In this case, we’ll use SUM():
SELECT SalesOrderID, SUM(OrderQty)
FROM Sales.SalesOrderDetail