1. Basic Nested Queries and Views
Jason C. H. Chen, Ph.D.
Professor of MIS
School of Business
Gonzaga University
Spokane, WA USA

2. Objectives
Learn what is nested query and how to create basic nested SQL queries
Learn what is database views and how/why to create views
Help you to make your job much easier on working with Queries for MVC mini-case (Phase III)

3. Subqueries and Their Uses
Subquery – a query nested inside another query
Used when a query is based on an unknown value
Requires SELECT and FROM clauses
Must be enclosed in parentheses
Place on right side of comparison operator
We will study more in chapter 12

4. You need to run the following command to make the example work:
Refresh Database
You need to run the following command to make the example work:
Download files from NW_CW folder
@ c:\oradata\NW_CW\northwoods.sql
Open the following script file:
NW_CW&Nested_Query.sql

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
that return one value
that returns one value

6. Subquery Returns Single Value Query: List all students who have the same S_CLASS value as student Jorge Perez.
SELECT s_last, s_first
FROM student
WHERE s_class =
You need to run the following command to make the example work:
@ c:\oradata\NW_CW\northwoods.sql
‘SR’;
It is impossible for us to manually examine the value (e.g., ‘SR’) that we should put in. What is a right way?
S_LAST S_FIRST
Jones Tammy
Perez Jorge

7. Subquery Returns Single Value Query: List all students who have the same S_CLASS value as student Jorge Perez.
SELECT s_last, s_first
FROM student
WHERE s_class =
You need to run the following command to make the example work:
@ c:\oradata\NW_CW\northwoods.sql
SELECT s_class
FROM student
WHERE s_last = 'Perez'
AND s_first = 'Jorge‘ ; ( )
S_LAST S_FIRST
Jones Tammy
Perez Jorge

8. Nested Query Where Subquery Returns Multiple Values
Syntax:
SELECT column1, column2, …
FROM table1, table2, …
WHERE join conditions
AND search_column1 IN
(SELECT column1
FROM table1, table2, …
WHERE search and
join conditions)
Subquery
that returns
multiple
values

9. Nested Query Where Subquery Returns Multiple Values
Display the names of all students who have enrolled in the same course sections as Jorge Perez.
SELECT DISTINCT s_last, s_first
FROM student, enrollment
WHERE student.s_id = enrollment.s_id
AND c_sec_id IN
(SELECT c_sec_id
FROM student, enrollment
WHERE student.s_id = enrollment.s_id
AND s_last = 'Perez'
AND s_first = 'Jorge');
S_LAST S_FIRST
Johnson Lisa
Jones Tammy
Marsh John
Perez Jorge

10. Database Views Table-1 Table-2 Table-N database Database view Query
A database view is a logical (virtual) table based on a query.
Table-1
Table-2
Table-N
It does not store data, but presents it in a format different from the one in which it is stored in the underlying tables.
With the database view, you can view database records, but you can’t insert new records or modify or delete exiting records.
database
Single view table
Database view
Query
Output:
Report, Graphs

11. Views
Permanent objects (logical or virtual table) that store queries but ____ data
Based on a source query that:
can specify a subset of a single table’s fields or records
can join multiple tables
Two purposes
Reduce complex query requirements
Restrict users’ access to sensitive data (enforce security; user has access to view but not underlying table) no

12. Creating and Using Database Views
Views can be updateable if:
SELECT clause contains only fieldnames, no functions or calculations
Can ____ contain the ORDER BY, DISTINCT, or GROUP BY clauses, group functions, or set operators
search condition cannot contain a nested query
Views are used like tables for selecting, inserting, updating and deleting data (only updatable views can be modified)
not

13. Database View (cont.) SELECT view_name FROM user_views;
SELECT view_name FROM ALL_VIEWS
WHERE owner=‘SYSTEM’;
DROP VIEW <view_name>;
CREATE OR REPLACE VIEW <view_name> AS
<view query specification>;

14. Database View What are gains? Anything New?
CREATE VIEW <view_name> AS
<view query specification>;
SELECT view_name FROM user_views;
e.g.,
CREATE OR REPLACE VIEW customers_vu AS
SELECT customer#, lastname, firstname, region
FROM customers;
SELECT * FROM customers_vu;
DESCRIBE customers_vu;
DELETING VIEWS
DROP VIEW viewname;
DROP VIEW customers_vu;
Anything New?
--EXTRA for VIEW
SELECT firstname || ' ' || lastname AS Name, order#, orderdate
FROM customers_vu, orders
WHERE customers_vu.customer# = orders.customer#
ORDER BY Name;
CREATE OR REPLACE VIEW customers_vu2 (customer_no, Last_name, First_name, Region) AS
SELECT customer#, lastname, firstname, region
FROM customers;
SELECT * FROM customers_vu2;
DESCRIBE customers_vu2;

15. Database View (cont.) CREATE OR REPLACE VIEW book_info_view AS
SELECT title, contact, phone
FROM books, publisher
WHERE books.pubid = publisher.pubid;
Anything new on the book_info_view ?
Ans: The view is created using two tables
SELECT *
FROM book_info_view
ORDER by title;

16. Creating a Simple View
Only references one table – no group functions, GROUP BY clause, or expressions
-- chapter 13, Figure 13-4(b); p.477
CREATE VIEW books_inventory_vu
AS SELECT isbn, title, retail price
FROM books
WITH READ ONLY;
-- chapter 13, Figure 13-4; p.477
CREATE VIEW inventory
AS SELECT isbn, title, retail price
FROM books
WITH READ ONLY;
SELECT *
FROM inventory;
SELECT *
FROM books_inventory_vu;
Figure Selecting all records from the INVENTORY view
What happen to “Database” after “DROP” view?
DROP VIEW inventory;
SELECT *
FROM inventory;
SELECT *
FROM books;
-- REcreate inventory view

17. CREATE OR REPLACE VIEW book_vu AS SELECT isbn, title, category
FROM books
WHERE category = 'COOKING';
SELECT *
FROM book_vu;
SQL> SELECT *
2 FROM book_vu;
ISBN TITLE CATEGORY
COOKING WITH MUSHROOMS COOKING
THE WOK WAY TO COOK COOKING
See next slide for detailed process

18. Figure 13-1 View Processing

19. Database View (continued)
CREATE VIEW <view_name> AS
<view query specification>;
e.g.,
SELECT view_name FROM user_views;
CREATE OR REPLACE VIEW customers_view AS
SELECT customer#, Lastname, Firstname
FROM customers;
SELECT view_name FROM user_views;
SELECT * FROM customers_view;
What are gains?
DELETING VIEWS
DROP VIEW viewname;
DROP VIEW customers_view;
--more example for VIEW
SELECT Lastname, Firstname, order#
FROM customers_view cv, orders
WHERE cv.customer# = orders.customer#
AND cv.customer#=1001;

20. Advanced Database View (continued)
Using SET Operators (e.g., UNION, INTERSECT etc.)
Highly recommended for MVC mini-case
CREATE VIEW <view_name> AS
<view query specification>
<SET operator>
<view query specification>;
e.g.,
CREATE OR REPLACE VIEW customers_view (f-1, f-2, …) AS
SELECT <field-1> <field-2> …
FROM <table-1> <table-2> …
WHERE <condition>
SELECT <field-3> <field-4> …
FROM <table-3> <table-4> …
WHERE <condition>;
CREATE OR REPLACE VIEW merged_view (f-1, f-2, …) AS
SELECT FROM View_1
SELECT FROM View_2;
/*Note: View_1 and View_2 have been created previously. */
<SET Operator>
<SET Operator>

21. More to go …
More Nested Queries and Subqueries will be studied in chapter 12.
More views will be studied in chapter 13.
You will apply the “view” [an efficient way to produce the solution] to the mini case (MVC mini-case) that will be assigned later.