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Chapter 12 Subqueries and Merge Statements (up to p.451)

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 99258 USA chen@jepson.gonzaga.edu. FOCUS – most of them have been studied. Three topics (Learn up to p. 451): Single-Row subquery

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Chapter 12 Subqueries and Merge Statements (up to p.451)

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  1. Chapter 12Subqueries and Merge Statements(up to p.451) Jason C. H. Chen, Ph.D. Professor of MIS School of Business Gonzaga University Spokane, WA 99258 USA chen@jepson.gonzaga.edu

  2. FOCUS – most of them have been studied • Three topics (Learn up to p. 451): • Single-Row subquery • Multiple-Row subquery • Multiple-Column subquery • 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 FROMClause (Query#11 – revisit Query#7)

  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. 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. Refresh the Database • 1. Run the following script file • Start c:\oradata\chapter12\JLDB_Build_12.sql

  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 • Subqueryretrieves values that the main query’s search condition must match that return one value that return one value

  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. Types of Subqueries Table 12-1 Topics Covered in This Chapter

  9. I. Single-Row Subqueries • Can only return one result to the outer query • Operators include =, >, <, >=, <=, < >

  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” Next, plug in the “found cost” into the second query -- chapter 12, Figure 12-2; p.430 SELECT category, title, cost FROM books WHERE cost > 31.4 AND category = 'COMPUTER'; Figure 12-3 A single-row subquery

  11. Single-Row Subquery in a WHERE Clause Query1: List all computer books with a retail price higher than the book “Database Implementation” 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' Only one value should be returned from the inner query (SELECT cost FROM books WHERE title = 'DATABASE IMPLEMENTATION’)

  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. 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. Query3: List title of all books published by publisher of ‘Big Bear and Little Dove’ that generate more than the average profit 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); Figure 12-6 SELECT statement with two single-row subqueries

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

  17. Single-Row Subquery in a SELECT Clause Query5: Compare the price of each book in inventory against average price of all books in inventory. • Replicates subquery value for each row displayed Figure 12-8 Single-row subquery in a SELECT clause

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

  19. II. Multiple-Row Subqueries • Return more than one row of results • Require use of IN, ANY, ALL, or EXISTS operators

  20. ANY and ALL Operators • Combine with arithmetic operators Table 12-2 ALL and ANY Operator Combinations

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

  22. Multiple-Row Subquery in a WHERE Clause SQL> SELECT MAX(retail) 2 FROM books 3 GROUP BY category; MAX(RETAIL) ----------- 75.95 28.75 59.95 39.95 31.95 30.95 89.95 29.95 8 rows selected. Figure 12-10 Multiple-row subquery with the IN operator

  23. Multiple-Row Subquery in a WHERE Clause 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). Figure 12-10 Multiple-row subquery with the IN operator

  24. Multiple-Row Subquery in a WHERE Clause (cont.) Query8: List all books with a retail price less than the most expensive book in the Cooking category. Practice other examples with >ALL, <ALL and >ANY Figure 12-14 Using <ANY operator

  25. 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. Multiple-Row Subquery in a HAVING Clause Q: try to use “ANY” and see the result. Highest ---SUBQUERY SELECT SUM( quantity paideach) FROM customers JOIN orders USING (customer#) JOIN orderitems USING (order#) WHERE state = 'FL' GROUP BY order#; SUM(QUANTITY*PAIDEACH) ---------------------- 106.85 85.45 54.5 17.9 75.9 Figure 12-17 Multiple-row subquery in a HAVING clause

  26. ---SUBQUERY SELECT SUM( quantity paideach) FROM customers JOIN orders USING (customer#) JOIN orderitems USING (order#) WHERE state = 'FL' GROUP BY order#; SUM(QUANTITY*PAIDEACH) ---------------------- 106.85 85.45 54.5 17.9 75.9 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) ---------- ---------------------- 1000 19.95 1001 117.4 1002 111.9 1003 106.85 1004 170.9 1005 39.95 1006 54.5 1007 335.85 1008 39.9 1009 41.95 1010 55.95 Lowest ORDER# SUM(QUANTITY*PAIDEACH) ---------- ---------------------- 1011 85.45 1012 166.4 1013 55.95 1014 44 1015 19.95 1016 85.45 1018 75.9 1019 22 1020 19.95

  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. 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 52.85 COOKING 24.35 CHILDREN 34.45 LITERATURE 39.95 BUSINESS 31.95 FITNESS 30.95 FAMILY LIFE 55.975 SELF HELP 29.95 it is considered a new table of “a” Figure 12-19 Multiple-column subquery in a FROM clause

  29. Multiple-Column Subquery in a FROM Clause (2nd solution) Figure 12-20 Using a Join with a multiple-column subquery in the FROM clause

  30. 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) Retail might match “MAX” from a different category. Hoewver, this example returns the same answer as the right one. Returns multiple columns for evaluation Figure 12-10: Multiple-row subquery with the IN operator Figure 12-21 Multiple-column subquery in a WHERE clause

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

  33. SQL> select customer#, referred from customers; CUSTOMER# REFERRED ---------- ---------- 1001 1002 1003 1004 1005 1006 1007 1003 1008 1009 1003 1010 1011 1012 1013 1006 1014 1015 1016 1010 1017 1018 1019 1003 1020 20 rows selected. When a subquery might return NULL values, use NVL function Figure 12-23 Using the NVL function to handle NULL values

  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. 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. Nested Subqueries (continued) Query12: List the name of the customer who has ordered the most books on one order (not including multiple quantities of the same book). • Innermost is resolved first (A), then the second level (B), then the outer query (C) -- 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 1007 1012 Figure 12-27 Nested subqueries

  37. Subquery in a DML action Figure 12-28 An UPDATE statement using a subquery

  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. Homework - Hands-On Assignments Email me with one attachment (Oracle_ch12_Spool_Lname_Fname.) to: chen@jepson.gonzaga.edu with subject title of Bmis441-01_Oracle_ch12 (or Bmis441-02_Oracle_ch12) Read and Practice all examples on Chapters 12 • 1. 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 email the file (Oracle_ch12_Spool_Lname_Fname.txt) to me by the midnight before the next class.

  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) Email me with the spooled file (.txt) with attachment to: chen@jepson.gonzaga.edu with subject title of Bmis441-01_Oracle_ch12 (or Bmis441-02_Oracle_ch12)

  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 =, >, <, >=, <=, and <> • Multiple-row subqueries return more than one row of results

  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. 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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