1. Aggregations
Various Aggregation Functions
GROUP BY
HAVING

2. Aggregate Functions
SQL Provides several functions that act on columns to provide aggregates of the values in the column
Find them under the Programmability | Functions | System Functions | Aggregate Functions section of the Object Browser

3. Aggregate Functions (cont.)
Try these two queries (run them together)
SELECT Quantity, UnitPrice, Quantity * UnitPrice AS 'Total'
FROM [Order Details]
WHERE OrderID = 10609
SELECT SUM(Quantity) AS 'Total Units Sold',
SUM(Quantity * UnitPrice) AS 'Total Value Sold'

4. Aggregate Functions (cont.)
Aggregate functions return one row with the aggregate values as the columns
You must use AS to give a name to the aggregate column as it will not have a 'natural' name
Most useful functions
AVG ( ) − COUNT( )
MIN ( ) − MAX( )
SUM ( )
Statistical functions
STDEV ( ) − STDEVP ( )
VAR ( ) − VARP ( )

5. Aggregate Functions (cont.)
The argument of each aggregate function is usually a column name
It may be a calculated expression SUM (Quantity * UnitPrice) AS 'Total Value Sold'
The COUNT ( ) function returns the number of rows in the result set regardless of which column is used
SELECT COUNT (*) AS… is commonly used
When a WHERE clause limits the rows with a criteria the aggregations are performed only on the rows that meet the WHERE criteria
Aggregates can be used in multi-table queries

6. Aggregate Function Exercises
Find the last OrderID in the DB
Find out what the next ProductID will be without actually adding a product
Find out the value of the inventory on hand (based on sales price) of the discontinued products
Find out the average number of products sold whenever the product Carnarvon Tigers have been sold

7. Grouping in Aggregate Functions
Without grouping an aggregate query will return just one row
SELECT * FROM [Order Details] WHERE OrderID = 10523
SELECT Orders.OrderID, COUNT(*) AS 'Number of Products',
SUM(Quantity) AS 'Number of Items',
SUM(Quantity * UnitPrice * (1 - Discount)) AS 'Revenue'
FROM Orders INNER JOIN [Order Details]
ON Orders.OrderID = [Order Details].OrderID
WHERE DatePart(mm, OrderDate) = 5
AND DatePart(yy, OrderDate) = 1997
GROUP BY Orders.OrderID

8. GROUP BY
GROUP BY in an aggregate query gives one row for each distinct value of the fields in the GROUP BY clause
Multiple items in the GROUP BY list can create multiple levels of grouping
Add ProductID to the GROUP BY clause of the previous query GROUP BY OrderID, ProductID
Items are grouped by the first item, then by the second within the first, etc.

9. GROUP BY (cont.)
Any column in the SELECT clause that is not an aggregate expression must be listed in the GROUP BY expression
Run the previous query with the GROUP BY commented out
If you wish to display multiple non-aggregated columns then they all must be listed in the GroupBy
Rewrite the previous query to display not only the OrderID but also the Customer CompanyName
You can apply an ORDER BY clause on an aggregated column when using GROUP BY

10. GROUP BY Exercises
Find the total value of each product in inventory by category (based on selling price) by showing the CategoryID, the quantity on hand, and the total value
Show the total value of sales by sales rep during May Show the sales rep's name and value.

11. Run this query with and without the last line
HAVING
HAVING acts like a WHERE clause applied to the results of a GROUP BY aggregation
Run this query with and without the last line
SELECT Orders.OrderID, COUNT(*) AS 'Number of Products',
SUM(Quantity) AS 'Number of Items',
SUM(Quantity * UnitPrice * (1 - Discount)) AS 'Revenue'
FROM Orders INNER JOIN [Order Details]
ON Orders.OrderID = [Order Details].OrderID
WHERE DatePart(mm, OrderDate) = 5
AND DatePart(yy, OrderDate) = 1997
GROUP BY Orders.OrderID
HAVING SUM(Quantity) < 10

12. HAVING Exercises
Management wants to know the names of your best customers. Find the CompanyNames and value purchased for all customers who ordered more than $1,000 in merchandise in May of 1997.