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Exploring SQL Aggregate Functions and Grouping Queries

Learn about essential SQL aggregate functions and how to group and filter data effectively in queries. Practice exercises provided to enhance your understanding.

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Exploring SQL Aggregate Functions and Grouping Queries

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  1. Aggregations • Various Aggregation Functions • GROUP BY • HAVING

  2. Aggregate Functions • SQL Provides several functions thatact on columns to provideaggregates 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' FROM [Order Details] WHERE OrderID = 10609

  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 expressionSUM (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 queryGROUP 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 1997. Show the sales rep's name and value.

  11. 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.

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