Basic Nested Queries and Views

Dr. Chen, Oracle Database System (Oracle) 1

Basic Nested Queries and Views

Jason C. H. Chen, Ph.D.Professor of MIS

School of BusinessGonzaga University

Spokane, WA 99258 [email protected]


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)


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


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 valuethat returns

one value


Refresh the Northwoods University Database

• Create a new folder of nw_cw (under c:\oradata\) and download all script files from the Bb

• Re-run a script fileSQL> @ c:\oradata\NW_CW\northwoods.sql


Subquery Returns Single Value Query: List all students who have the same S_CLASS value as

student Jorge Perez.

SELECT s_last, s_firstFROM studentWHERE s_class =

S_LAST S_FIRST---------- ---------Jones TammyPerez Jorge

‘SR’;It is impossible for us to manually examine the value (e.g., ‘SR’) that we should put in. What is a right way?


Subquery Returns Single Value Query: List all students who have the same S_CLASS value as

student Jorge Perez.

SELECT s_class FROM student WHERE s_last = 'Perez' AND s_first = 'Jorge‘ ;

SELECT s_last, s_firstFROM studentWHERE s_class =

(

)

You need to run the following command to make the example work:@ c:\oradata\NW_CW\northwoods.sql

S_LAST S_FIRST---------- ---------Jones TammyPerez Jorge


Nested Query WhereSubquery Returns Multiple Values

• Syntax:SELECT column1, column2, …FROM table1, table2, …WHERE join conditionsAND search_column1 IN

(SELECT column1FROM table1, table2, …WHERE search and join conditions)

Subquerythat returnsmultiple values


Nested Query WhereSubquery Returns Multiple Values

• Display the names of all students who have enrolled in the same course sections as Jorge Perez.

(SELECT c_sec_id FROM student, enrollment WHERE student.s_id = enrollment.s_id

AND s_last = 'Perez' AND s_first = 'Jorge');

SELECT DISTINCT s_last, s_firstFROM student, enrollmentWHERE student.s_id = enrollment.s_idAND c_sec_id IN

S_LAST S_FIRST--------------- -------------Johnson LisaJones TammyMarsh JohnPerez Jorge


Database Views

Table-1 Table-NTable-2

database

Single view table

Query

Output:Report, Graphs

Database view

A database view is a logical (virtual) table based on a query.

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 in the view.


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


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


Database View (cont.)

h SELECT view_name FROM user_views;h SELECT view_name FROM ALL_VIEWS WHERE owner=‘SYSTEM’;h DROP VIEW <view_name>;h CREATE OR REPLACE VIEW <view_name> AS <view query specification>;



What are gains?

--EXTRA for VIEWSELECT firstname || ' ' || lastname AS Name, order#, orderdateFROM customers_vu, ordersWHERE customers_vu.customer# = orders.order#ORDER BY Name;

i DELETING VIEWSDROP VIEW viewname;

DROP VIEW customers_vu;

CREATE VIEW <view_name> AS <view query specification>; e.g., SELECT view_name FROM user_views;

CREATE OR REPLACE VIEW customers_vu ASSELECT customer#, lastname, firstname, regionFROM customers;


CREATE OR REPLACE VIEW book_info_view ASSELECT title, contact, phoneFROM books, publisherWHERE books.pubid = publisher.pubid;

SELECT *FROM book_info_viewORDER by title;


Anything new on the book_info_view ?

Ans: The view is created using two tables

DROP VIEW book_info_view;


Creating a Simple View

• Only references one table – no group functions, GROUP BY clause, or expressions

Figure 13-4 Selecting all records from the INVENTORY view

-- chapter 13, Figure 13-4(b); p.477-- a more meaningful view nameCREATE VIEW books_inventory_vuAS SELECT isbn, title, retail price FROM booksWITH READ ONLY;SELECT *FROM books_inventory_vu;

-- chapter 13, Figure 13-4; p.477CREATE VIEW inventoryAS SELECT isbn, title, retail price FROM booksWITH READ ONLY;

SELECT *FROM inventory;

DROP VIEW inventory;

SELECT *FROM inventory;

What happen to “Database” after “DROP” view?

SELECT *FROM books;-- You may REcreate inventory view


CREATE OR REPLACE VIEW book_vu ASSELECT isbn, title, categoryFROM booksWHERE category = 'COOKING';

SELECT * FROM book_vu;

SQL> SELECT * 2 FROM book_vu;

ISBN TITLE CATEGORY---------- ------------------------------ ---------3437212490 COOKING WITH MUSHROOMS COOKING0299282519 THE WOK WAY TO COOK COOKING

See next slide for detailed process


Figure 13-1 View Processing


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 ASSELECT customer#, Lastname, FirstnameFROM customers;

SELECT view_name FROM user_views;SELECT * FROM customers_view;

What are gains?

--more example for VIEWSELECT Lastname, Firstname, cv.customer#, order#FROM customers_view cv, ordersWHERE cv.customer# = orders.customer#AND cv.customer#=1001;

i DELETING VIEWSDROP VIEW viewname;

DROP VIEW customers_view;


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.


• END OF VIEWS

Documents

Basic Nested Queries and Views