Aggregations Various Aggregation Functions GROUP BY HAVING

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

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'

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 ( )

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

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

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

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.

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

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 1997. Show the sales rep's name and value.

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

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.