Chapter 12
Subqueries and Merge
Statements
(up to p.473)
Jason C. H. Chen, Ph.D.
Professor of MIS
School of Business
Gonzaga University
Spokane, WA 99258 USA
chen@gonzaga.edu
Dr. Chen, Oracle Database System (Oracle)
1
FOCUS – most of them have been studied
• Three topics (Learn up to p. 473):
– 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 followings:
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)
Dr. Chen, Oracle Database System (Oracle)
2
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
Dr. Chen, Oracle Database System (Oracle)
3
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
Dr. Chen, Oracle Database System (Oracle)
4
Refresh the Database
• 1. Run the following script file
– Start c:\oradata\chapter12\JLDB_Build_12.sql
Dr. Chen, Oracle Database System (Oracle)
5
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
Dr. Chen, Oracle Database System (Oracle)
that return
thatvalue
return
one
one value
6
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
Dr. Chen, Oracle Database System (Oracle)
7
Types of Subqueries
Table 12-1
Topics Covered in This Chapter
Dr. Chen, Oracle Database System (Oracle)
8
I. Single-Row Subqueries
• Can only return one result to the outer query
• Operators include =, >, <, >=, <=, < >
Dr. Chen, Oracle Database System (Oracle)
9
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
Figure 12-3 A single-row subquery
Dr. Chen, Oracle Database System (Oracle)
-- chapter 12, Figure 12-2; p.452
SELECT category, title, cost
FROM books
WHERE cost > 31.4
AND category = 'COMPUTER';
10
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.453
SELECT category, title, cost
FROM books
WHERE cost >
(SELECT cost
FROM books
WHERE title = 'DATABASE
IMPLEMENTATION’)
AND category = 'COMPUTER'
Dr. Chen, Oracle Database System (Oracle)
Only one value
should be
returned from
the inner
query
11
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
Dr. Chen, Oracle Database System (Oracle)
12
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);
Dr. Chen, Oracle Database System (Oracle)
Figure 12-5 Query to determine the title of the most expensive book
13
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 =
(SELECT pubid
FROM books
WHERE title = 'BIG
BEAR AND LITTLE DOVE')
AND retail-cost >
(SELECT AVG(retail-cost)
FROM books);
Dr. Chen, Oracle Database System (Oracle)
Figure 12-6 SELECT statement with two singlerow subqueries
14
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.
Dr. Chen, Oracle Database System (Oracle)
15
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
#3
Figure 12-7
Dr. Chen, Oracle Database System (Oracle)
#1
Single-row subquery nested in a HAVING clause
16
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
Dr. Chen, Oracle Database System (Oracle)
17
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
Dr. Chen, Oracle Database System (Oracle)
18
II. Multiple-Row Subqueries
• Return more than one row of results
(multiple values)
• Require use of IN, ANY, ALL, or EXISTS
operators
• Might have potential problems (what and
how to solve it)
Dr. Chen, Oracle Database System (Oracle)
19
ANY and ALL Operators
• Combine with arithmetic operators
Table 12-2
ALL and ANY Operator Combinations
Dr. Chen, Oracle Database System (Oracle)
20
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
1. Determine the (retail) 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
-- chapter 12, Figure 12-10; p.460
SELECT title, retail, category
FROM books
WHERE retail IN (SELECT MAX(retail) # this part will
FROM books be executed first
GROUP BY category)
ORDER BY category;
Dr. Chen, Oracle Database System (Oracle)
Note: Could use IN operator or =ANY
21
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
Figure 12-10
Multiple-row subquery with the IN operator
Dr. Chen, Oracle Database System (Oracle)
8 rows selected.
22
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?
Figure 12-10
Multiple-row subquery with the IN operator
Dr. Chen, Oracle Database System (Oracle)
A: Use
“Multiplecolumn”
subquery (see
next session).
23
Query#7 - revisit
• Revisit in Query#11
Dr. Chen, Oracle Database System (Oracle)
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.
< 28.75
Figure 12-14
Dr. Chen, Oracle Database System (Oracle)
Practice other
examples with
>ALL, <ALL
and >ANY
Using <ANY operator
25
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.
Q: try to
use
“ANY”
and see
the result
(next
slide)
---SUBQUERY
SELECT SUM( quantity*paideach)
FROM customers JOIN orders USING (customer#)
JOIN orderitems USING (order#)
WHERE state = 'FL'
GROUP BY order#;
Highest
Figure 12-17
Multiple-row subquery in a HAVING clause
Dr. Chen, Oracle Database System (Oracle)
SUM(QUANTITY*PAIDEACH)
---------------------106.85
85.45
54.5
17.9
75.9
26
---SUBQUERY
SQL> SELECT order#, SUM( quantity * paideach)
SELECT SUM( quantity*paideach)
2 FROM orderitems
FROM customers JOIN orders USING (customer#)
3 HAVING SUM( quantity * paideach) >
JOIN orderitems USING (order#)
4
ANY (SELECT SUM( quantity * paideach) WHERE state = 'FL'
GROUP BY order#;
5
FROM customers JOIN orders USING
(customer#)
SUM(QUANTITY*PAIDEACH)
6
JOIN orderitems USING (order#)
---------------------7
WHERE state = 'FL'
106.85
8
GROUP BY order#)
85.45
Lowest
54.5
9 GROUP BY order#;
17.9
75.9
ORDER# SUM(QUANTITY*PAIDEACH)
---------- ---------------------1000
19.95
-- continued from last page
1001
117.4
ORDER# SUM(QUANTITY*PAIDEACH)
1002
111.9
---------- ---------------------1003
106.85
1011
85.45
1004
170.9
1012
166.4
1005
39.95
1013
55.95
1006
54.5
1014
44
1007
335.85
1015
19.95
1008
39.9
1016
85.45
1009
41.95
1018
75.9
1010
55.95
1019
22
1020
19.95
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
Dr. Chen, Oracle Database System (Oracle)
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
Dr. Chen, Oracle Database System (Oracle)
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
Dr. Chen, Oracle Database System (Oracle)
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
(by luck) as
the right
one.
Returns
multiple
columns
for
evaluation
Figure 12-10: Multiple-row subquery with the IN operator
Dr. Chen, Oracle Database System (Oracle)
Figure 12-21 Multiple-column subquery in a WHERE clause
31
NVL Function (also see p. 377)
• 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.
Dr. Chen, Oracle Database System (Oracle)
32
NULL Values
Query12: 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.471
SELECT customer#
FROM customers
WHERE referred = (SELECT
referred
FROM customers
WHERE customer# =
1005);
SELECT referred
FROM customers
WHERE customer# = 1005;
Figure 12-22 Flawed query: NULL results from a subquery
Dr. Chen, Oracle Database System (Oracle)
REFERRED
----------
33
When a subquery might return NULL values, use NVL function
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
Figure 12-23
Using the NVL function to handle NULL values
Dr. Chen, Oracle Database System (Oracle)
20 rows selected.
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.
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.
Dr. Chen, Oracle Database System (Oracle)
36
Nested Subqueries (continued)
Query13: 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).
• 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#));
Dr. Chen, Oracle Database System (Oracle)
Figure 12-27
Nested subqueries
(A): returns 4
(B): returns
1007
1012
37
Subquery in a DML action
Figure 12-28
Dr. Chen, Oracle Database System (Oracle)
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.492)
Dr. Chen, Oracle Database System (Oracle)
3.
SELECT order#
FROM orders
WHERE shipstate = (SELECT shipstate
FROM orders
WHERE order# = 1014);
39
Homework - Hands-On Assignments
Upload the spooled file (*.txt) to the Bb (under
“Assignments & Projects”) by the deadline.
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. 492-493) 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.
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 the Bb (under
“Assignments & Projects”) by the deadline.
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 =, >, <, >=, <=, and <>
• Multiple-row subqueries return more than one row of results
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 multiplecolumn 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
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
Dr. Chen, Oracle Database System (Oracle)
44