1. Subqueries CIT 381 2001 Oct 26

2. Nesting A query can be put inside another query Most commonly in the WHERE clause Sometimes in the FROM clause (depending on the software) This subquery is executed first (if possible)

3. % Find the average sales of the Sales Reps SELECT AVG(sales) FROM salesreps; Expr1000 $289,353.20 % Find those Sales Reps whose sales exceed the average SELECT name, sales FROM salesreps WHERE sales > (SELECT AVG(sales) FROM salesreps); namesales Dan Roberts$305,673.00 Sue Smith$474,050.00 Bill Adams$367,911.00 Sam Clark$299,912.00 Larry Fitch$361,865.00 Mary Jones$392,725.00

4. % Find the Sales Rep with maximum sales Two ways: SELECT name, sales FROM salesreps WHERE sales = (SELECT MAX(sales) FROM salesreps); -or- SELECT name, sales FROM salesreps WHERE sales >= ALL (SELECT sales FROM salesreps); namesales Sue Smith$474,050.00

5. Correlated vs Uncorrelated The previous subqueries did not depend on anything outside the subquery …and thus need to be executed just once. There are called uncorrelated. A correlated subquery depends on data from the outer query … and thus has to be executed for each row of the outer table(s)

6. Correlated Example % For each office, find the Sales Rep with the most sales in that office. SELECT s_out.name, s_out.sales FROM salesreps s_out WHERE s_out.sales= (SELECT MAX(s_in.sales) FROM salesreps s_in WHERE s_in.rep_office=s_out.rep_office) namesales Dan Roberts$305,673.00 Sue Smith$474,050.00 Bill Adams$367,911.00 Nancy Angelli$186,042.00 Mary Jones$392,725.00

7. Macho SQL SELECT s1.rep_office, s1.name, s1.sales FROM salesreps AS s1, (SELECT rep_office, MAX(sales) AS ms FROM salesreps GROUP BY rep_office) AS s2 WHERE s1.sales=s2.ms AND s1.rep_office=s2.rep_office rep_officenamesales 12Dan Roberts$305,673.00 21Sue Smith$474,050.00 13Bill Adams$367,911.00 22Nancy Angelli$186,042.00 11Mary Jones$392,725.00 The subquery could be viewed as a table and put in the FROM clause.

8. In the previous lecture we wrote a query for % Get the average and total sales of those Reps with over $30k in total sales. Someone asked: “How do we get the sum of those total sales?” That is, sum the sales of all those who exceed $30,000 in sales. We can (1) use a subquery in the FROM clause Or (2) use a TEMP table. (Solution 2 not in Access??)

9. Solution 1 SELECT SUM(TotalSales) FROM (SELECT s.name, AVG(o.amount) as AvgSales, SUM(o.amount) AS TotalSales FROM salesreps s, orders o WHERE s.empl_num=o.rep GROUP BY s.name HAVING SUM(o.amount)>=30000); Expr1000 $165,350.00

10. Subqueries as Sets A subquery can be viewed as a set (in the WHERE clause) Viewed as a table in the FROM clause (more rare and less supported) As sets, can use set operations IN, EXISTS, ANY, ALL

11. Simulate a Join % Find all managers. SELECT name FROM salesreps WHERE empl_num IN (SELECT manager FROM salesreps) You could do this as a self-join. That would be more efficient, because join computations are optimized. name Dan Roberts Bob Smith Sam Clark Larry Fitch

12. Tricky NOT IN % Find those people who are not managers. SELECT name FROM salesreps WHERE empl_num NOT IN (SELECT manager FROM salesreps) OOPS (try it). This gives no names. Obviously the wrong answer.

13. Look at contents of SALESREPS EMPL_NUMNAMEAGEREP_OFFICETITLEHIRE_DATEMANAGERQUOTASALES 101Dan Roberts4512Sales Rep20-Oct-86104$300,000.00$305,673.00 102Sue Smith4821Sales Rep10-Dec-86108$350,000.00$474,050.00 103Paul Cruz2912Sales Rep01-Mar-87104$275,000.00$286,775.00 104Bob Smith3312Sales Mgr19-May-87106$200,000.00$142,594.00 105Bill Adams3713Sales Rep12-Feb-88104$350,000.00$367,911.00 106Sam Clark5211VP Sales14-Jun-88$275,000.00$299,912.00 107Nancy Angelli4922Sales Rep14-Nov-88108$300,000.00$186,042.00 108Larry Fitch6221Sales Mgr12-Oct-89106$350,000.00$361,865.00 109Mary Jones3111Sales Rep12-Oct-89106$300,000.00$392,725.00 110Tom Snyder41Sales Rep13-Jan-90101$75,985.00

14. Tricky NULLS again One of the managers is NULL. That could be anybody (so we can’t say anyone is not in there). SELECT name FROM salesreps WHERE empl_num NOT IN (SELECT manager FROM salesreps WHERE manager IS NOT NULL) name Sue Smith Paul Cruz Bill Adams Nancy Angelli Mary Jones Tom Snyder