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Dr. Chen, Oracle Database System (Oracle) 1 Chapter 12 Subqueries and Merge Statements (up to p.451) Jason C. H. Chen, Ph.D. Professor of MIS School of.

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Presentation on theme: "Dr. Chen, Oracle Database System (Oracle) 1 Chapter 12 Subqueries and Merge Statements (up to p.451) Jason C. H. Chen, Ph.D. Professor of MIS School of."— Presentation transcript:

1 Dr. Chen, Oracle Database System (Oracle) 1 Chapter 12 Subqueries and Merge Statements (up to p.451) Jason C. H. Chen, Ph.D. Professor of MIS School of Business Gonzaga University Spokane, WA USA

2 Dr. Chen, Oracle Database System (Oracle) 2 FOCUS – most of them have been studied Three topics (Learn up to p. 451): –Single-Row subquery Subquery returns “single” value –Multiple-Row subquery Subquery returns “multiple” values –Multiple-Column subquery For “matching” return values on different subjects We will focus on the following: Single-Row Subquery in a HAVING Clause (Query#4) Multiple-Row Subquery in “IN” (Query#7) Multiple-Column Subquery in a FROM Clause (Query#11 – revisit Query#7)

3 Dr. Chen, Oracle Database System (Oracle) 3 Objectives Determine when using a subquery is appropriate Identify which clauses can contain subqueries Distinguish between an outer query and a subquery Use a single-row subquery in a WHERE clause Use a single-row subquery in a HAVING clause Use a single-row subquery in a SELECT clause Distinguish between single-row and multiple-row comparison operators Use a multiple-row subquery in a WHERE clause

4 Dr. Chen, Oracle Database System (Oracle) 4 Objectives (continued) Use a multiple-row subquery in a HAVING clause Use a multiple-column subquery in a WHERE clause Create an inline view using a multiple-column subquery in a FROM clause Compensate for NULL values in subqueries Distinguish between correlated and uncorrelated subqueries Nest a subquery inside another subquery Use a subquery in a DML action Process multiple DML actions with a MERGE statement

5 Dr. Chen, Oracle Database System (Oracle) 5 Refresh the Database 1. Run the following script file –Start c:\oradata\chapter12\JLDB_Build_12.sql

6 Dr. Chen, Oracle Database System (Oracle) 6 Creating Nested Queries Used to select results based on the result of a query Consists of a main query and one or more subqueries. –Main/outer query: first query that appears in the SELECT command –Subquery retrieves values that the main query’s search condition must match that return one value

7 Dr. Chen, Oracle Database System (Oracle) 7 Subqueries and Their Rules/Uses Subquery – a query nested inside another query and used when a query is based on an unknown value. A subquery must be complete query in itself – it requires SELECT and FROM clauses A subquery, except one in the FROM clause, can’t have an ORDER BY clause (on the outer query’s last clause). Must be enclosed in parentheses to separate it from the outer/main query Place on right side of comparison operator

8 Dr. Chen, Oracle Database System (Oracle) 8 Types of Subqueries Table 12-1 Topics Covered in This Chapter

9 Dr. Chen, Oracle Database System (Oracle) 9 I. Single-Row Subqueries Can only return one result to the outer query Operators include =, >, =,

10 Dr. Chen, Oracle Database System (Oracle) 10 Query: List all computer books with a retail price higher than the book “Database Implementation” First, find out the “cost” of book “Database Implementation” -- chapter 12, Figure 12-2; p.430 SELECT category, title, cost FROM books WHERE cost > 31.4 AND category = 'COMPUTER'; Next, plug in the “found cost” into the second query Figure 12-3 A single-row subquery

11 Dr. Chen, Oracle Database System (Oracle) 11 Single-Row Subquery in a WHERE Clause Subquery – a query nested inside another query and used when a query is based on an unknown value. -- chapter 12, Figure 12-3; p.431 SELECT category, title, cost FROM books WHERE cost > AND category = 'COMPUTER' (SELECT cost FROM books WHERE title = 'DATABASE IMPLEMENTATION’) Query1: List all computer books with a retail price higher than the book “Database Implementation” Only one value should be returned from the inner query

12 Dr. Chen, Oracle Database System (Oracle) 12 Query2: List title of the most expensive book sold by JustLee Books (incorrect example) Figure 12-4 Flawed query: attempt to determine the book with the highest retail value

13 Dr. Chen, Oracle Database System (Oracle) 13 Query2: List title of the most expensive book sold by JustLee Books (a correct example) SELECT title, retail FROM books WHERE retail = (SELECT MAX(retail) FROM books); Figure 12-5 Query to determine the title of the most expensive book

14 Dr. Chen, Oracle Database System (Oracle) 14 Query3: List title of all books published by publisher of ‘Big Bear and Little Dove’ that generate more than the average profit Figure 12-6 SELECT statement with two single- row subqueries Tasks: two unknown values: 1) the pubid of ‘Big Bear and Little Dove’, 2) the average profit of the all books. SELECT isbn, title FROM books WHERE pubid = AND retail-cost > (SELECT pubid FROM books WHERE title = 'BIG BEAR AND LITTLE DOVE') (SELECT AVG(retail-cost) FROM books);

15 Dr. Chen, Oracle Database System (Oracle) 15 Query4: List all book categories returning a higher average profit than the ‘Literature’ category. Three steps are needed for this task: 1. calculate the average profit for all ‘Literature’ books. 2. calculate the average profit for each category. 3. compare the average profit for each category with the average profit for the ‘Literature’ category.

16 Dr. Chen, Oracle Database System (Oracle) 16 Single-Row Subquery in a HAVING Clause Required when returned value is compared to grouped data Figure 12-7 Single-row subquery nested in a HAVING clause Query4: List all book categories returning a higher average profit than the ‘Literature’ category. #1 #2 #3

17 Dr. Chen, Oracle Database System (Oracle) 17 Single-Row Subquery in a SELECT Clause Replicates subquery value for each row displayed Figure 12-8 Single-row subquery in a SELECT clause Query5: Compare the price of each book in inventory against average price of all books in inventory.

18 Dr. Chen, Oracle Database System (Oracle) 18 Single-Row Subquery in a SELECT Clause (coni.) Replicates subquery value for each row displayed Figure 12-9 Use a subquery in a calculation in the SELECT clause Query6: List the difference between each book price and the average.

19 Dr. Chen, Oracle Database System (Oracle) 19 II. Multiple-Row Subqueries Return more than one row of results Require use of IN, ANY, ALL, or EXISTS operators

20 Dr. Chen, Oracle Database System (Oracle) 20 ANY and ALL Operators Combine with arithmetic operators Table 12-2 ALL and ANY Operator Combinations

21 Dr. Chen, Oracle Database System (Oracle) 21 Multiple-Row Subquery in a WHERE Clause Note: Could use IN operator or =ANY Query7: List book titles, retail value and category that match the highest retail value for any book category -- chapter 12, Figure 12-10; p.438 SELECT title, retail, category FROM books WHERE retail ORDER BY category; 1.Determine the price of the most expensive book in each category 2.The maximum retail price in each category is to the WHERE clause of the outer query (more than one) 3.The outer query compares each book’s price to the prices from #2 4.If a book’s rail price matches one of the prices returned, the book’s title, retail price, and category are displayed in the output (SELECT MAX(retail) FROM books GROUP BY category) # this part will be executed first IN

22 Dr. Chen, Oracle Database System (Oracle) 22 Multiple-Row Subquery in a WHERE Clause Figure Multiple-row subquery with the IN operator SQL> SELECT MAX(retail) 2 FROM books 3 GROUP BY category; MAX(RETAIL) rows selected.

23 Dr. Chen, Oracle Database System (Oracle) 23 Multiple-Row Subquery in a WHERE Clause Figure Multiple-row subquery with the IN operator Q: what might be a problem on this query? A: retail might match “MAX” from a different category. Q: how to solve the problem? A: Use “Multiple- column” subquery (see next session).

24 Dr. Chen, Oracle Database System (Oracle) 24 Multiple-Row Subquery in a WHERE Clause (cont.) Figure Using ALL, ANY

25 Dr. Chen, Oracle Database System (Oracle) 25 Multiple-Row Subquery in a HAVING Clause Figure Multiple-row subquery in a HAVING clause Query9: Check whether any customer’s recently placed order has a total amount due greater than the total amount due for every order placed recently by customers in Florida. ---SUBQUERY SELECT SUM( quantity*paideach) FROM customers JOIN orders USING (customer#) JOIN orderitems USING (order#) WHERE state = 'FL' GROUP BY order#; SUM(QUANTITY*PAIDEACH) Q: try to use “ANY” and see the result. Highest

26 Dr. Chen, Oracle Database System (Oracle) 26 SQL> SELECT order#, SUM( quantity * paideach) 2 FROM orderitems 3 HAVING SUM( quantity * paideach) > 4 ANY (SELECT SUM( quantity * paideach) 5 FROM customers JOIN orders USING (customer#) 6 JOIN orderitems USING (order#) 7 WHERE state = 'FL' 8 GROUP BY order#) 9 GROUP BY order#; ORDER# SUM(QUANTITY*PAIDEACH) continued from last page ORDER# SUM(QUANTITY*PAIDEACH) SUBQUERY SELECT SUM( quantity*paideach) FROM customers JOIN orders USING (customer#) JOIN orderitems USING (order#) WHERE state = 'FL' GROUP BY order#; SUM(QUANTITY*PAIDEACH) Lowest

27 Dr. Chen, Oracle Database System (Oracle) 27 III. Multiple-Column Subqueries Return more than one column in results Creates a temporary table (or inline view) Can return more than one column to the outer query (more than one column from the subquery) Column list on the left side of operator must be in parentheses Use the IN operator for WHERE and HAVING clauses

28 Dr. Chen, Oracle Database System (Oracle) 28 Multiple-Column Subquery in a FROM Clause Figure Multiple-column subquery in a FROM clause Query10: List all books that have a higher-than-average selling price compared with other books in the same category. SELECT category, AVG(retail) cataverage FROM books GROUP BY category; CATEGORY CATAVERAGE COMPUTER COOKING CHILDREN LITERATURE BUSINESS FITNESS FAMILY LIFE SELF HELP it is considered a new table of “a”

29 Dr. Chen, Oracle Database System (Oracle) 29 Multiple-Column Subquery in a FROM Clause (2 nd solution) Figure Using a Join with a multiple-column subquery in the FROM clause

30 Dr. Chen, Oracle Database System (Oracle) 30 Multiple-Column Subquery in a WHERE Clause Figure Multiple-column subquery in a WHERE clause Query11(#7): List book titles, retail value and category that match the highest retail value for any book category - revisit (the right solution) Figure 12-10: Multiple-row subquery with the IN operator Retail might match “MAX” from a different category. Hoewver, this example returns the same answer (by luck) as the right one. Returns multiple columns for evaluation

31 Dr. Chen, Oracle Database System (Oracle) 31 NVL Function (also see p. 359) The NVL function is to address problems caused when performing arithmetic operations with fields that might contain NULL values. NULL value is the absence of data, not a blank space or a zero. NVL(x,y) where y represents the value to substitute if x is NULL.

32 Dr. Chen, Oracle Database System (Oracle) 32 Figure Flawed query: NULL results from a subquery NULL Values Query11: List all customers (customer#) who referred customer 1005 has referred any other customers to JustLee Books. Q: what causes the problem? SELECT referred FROM customers WHERE customer# = 1005; REFERRED chapter 12, Figure 12-22; p.449 SELECT customer# FROM customers WHERE referred = (SELECT referred FROM customers WHERE customer# = 1005);

33 Dr. Chen, Oracle Database System (Oracle) 33 When a subquery might return NULL values, use NVL function Figure Using the NVL function to handle NULL values SQL> select customer#, referred from customers; CUSTOMER# REFERRED rows selected.

34 Dr. Chen, Oracle Database System (Oracle) 34 IV. Uncorrelated Subqueries Processing sequence –Inner query is executed first –Result is passed to outer query –Outer query is executed Most of subqueries we studied are uncorrelated subqueries.

35 Dr. Chen, Oracle Database System (Oracle) 35 Nested Subqueries Maximum of 255 subqueries if nested in the WHERE clause No limit if nested in the FROM clause Innermost subquery is resolved first, then the next level, etc.

36 Dr. Chen, Oracle Database System (Oracle) 36 Nested Subqueries (continued) Innermost is resolved first (A), then the second level (B), then the outer query (C) Figure Nested subqueries Query12: List the name of the customer who has ordered the most books (4 books in this case) on one order (not including multiple quantities of the same book). -- chapter 12, Figure 12-27; p.454 SELECT customer#, lastname, firstname FROM customers JOIN orders USING (customer#) WHERE order# IN (SELECT order# FROM orderitems GROUP BY order# HAVING COUNT(*) = (SELECT MAX(COUNT(*)) FROM orderitems GROUP BY order#)); (A): returns 4 (B): returns

37 Dr. Chen, Oracle Database System (Oracle) 37 Subquery in a DML action Figure An UPDATE statement using a subquery

38 Dr. Chen, Oracle Database System (Oracle) 38 Exercises Practice all the examples in the text. A Script file is available on the Bb (file name: ch12Queries.sql) After completing all examples, do the HW. In-class Exercise #3 (p.468)

39 Dr. Chen, Oracle Database System (Oracle) 39 Homework - Hands-On Assignments Read and Practice all examples on Chapters Run the script files (in the folder \oradata\chapter12\): JLDB_Build_12.sql 2. Read Oracle assignment and create a script file Oracle_ch12_Lname_Fname.sql for questions (#1,2, 5, 10; p. 468) on “Hands-on Assignments”. Use appropriate COLUMN statements to produce readable outputs 3. Execute and test one problem at a time and make sure they are all running successfully. 4. When you done, spool the script files (see next slide for spooling instructions) and UPLOAD the file (Oracle_ch12_Spool_Lname_Fname.txt) to the Bb by the midnight before the next class. me with one attachment (Oracle_ch12_Spool_Lname_Fname.) to: with subject title of Bmis441-01_Oracle_ch12 (or Bmis441-02_Oracle_ch12)

40 Dr. Chen, Oracle Database System (Oracle) 40 How to Spool your Script and Output Files After you tested the script file of Oracle_ch12_Lname_Fname.sql successfully, follow the instructions below to spool both script and output files: Step 0. Run the following script file from SQL*Plus (since you have created JLDB tables) –Start c:\oradata\chapter12\JLDB_Build_12.sql 1. type the following on SQL> –Spool c:\oradata\Oracle_ch12_Spool_Lname_Fname.txt (make sure your name is entered) 2. open Oracle_ch12_Lname_Fname.sql that you already tested 3. copy and paste all the SQL commands (including all comments) to the SQL*PLUS 4. type Spool Off on the SQL> The output should contain your personal information, all SQL commands and their solution on the.txt file and saved in C: drive (oradata\ folder) UPLOAD the spooled file (.txt) to Bb.

41 Dr. Chen, Oracle Database System (Oracle) 41 Summary A subquery is a complete query nested in the SELECT, FROM, HAVING, or WHERE clause of another query –The subquery must be enclosed in parentheses and have a SELECT and a FROM clause, at a minimum Subqueries are completed first; the result of the subquery is used as input for the outer query A single-row subquery can return a maximum of one value Single-row operators include =, >, =, Multiple-row subqueries return more than one row of results

42 Dr. Chen, Oracle Database System (Oracle) 42 Summary (continued) Operators that can be used with multiple-row subqueries include IN, ALL, ANY, and EXISTS Multiple-column subqueries return more than one column to the outer query NULL values returned by a multiple-row or multiple- column subquery will not present a problem if the IN or =ANY operator is used Correlated subqueries reference a column contained in the outer query Subqueries can be nested to a maximum depth of 255 subqueries in the WHERE clause of the parent query

43 Dr. Chen, Oracle Database System (Oracle) 43 Summary (continued) With nested subqueries, the innermost subquery is executed first, then the next highest level subquery is executed, and so on, until the outermost query is reached A MERGE statement allows multiple DML actions to be conditionally performed while comparing data of two tables


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