Relational Model:
Advanced Topics
Chapter 4
Views
An individual user’s, or an application
program’s picture of the database.
You can think of it as a saved query.
It provides a measure of security – omitting
sensitive tables or fields.
Example
• This might be used for a salesperson’s
view:
CREATE VIEW Housewares AS
SELECT PartNum, Description, OnHand, Price
FROM Part
WHERE Class = ‘HW’
;
The salesperson could write this query of the view:
SELECT *
FROM Housewares
WHERE OnHand < 25
;
What really happens in the DBMS is:
SELECT PartNum, Description, OnHand, Price
FROM Part
WHERE Class = ‘HW’
AND OnHand < 25
;
• To the salesperson, it seems like he’s
using a table named Housewares.
• Any update to the Part table is
immediately visible in the Housewares
view (because it’s not really a table).
• In Access, to make a view, create & save
query.
– Forms & Reports can be driven by views.
CREATE VIEW Housewares (PNum, PDesc, OnHd,
Price) AS
SELECT PartNum, Description, OnHand, Price
FROM Part
WHERE Class = ‘HW’
;
This gives the salesperson a customized
view, with different field names he might
prefer.
Views can join tables
CREATE VIEW SalesCust (SNum, SLast, SFirst, CNum,
CName) AS
SELECT Rep.RepNum, LastName, FirstName, CustomerNum,
CustomerName
FROM Rep, Customer
WHERE Rep.RepNum = Customer.RepNum
;
A query might be written:
SELECT SNum, SLast, SFirst
FROM SalesCust
WHERE CNum = ‘282’;
An example of a query in code…
Indexes
• Like finding something quickly in a book.
• Indexing can speed up the database
performance.
• Happens behind the scenes – its used by
the DBMS, not the user.
• Most helpful where tables have thousands,
tens of thousands, or hundreds of
thousands of records..
CreditLimit Index
CreditLimit
RecordNum
$5,000
4,8
$7,500
1,3,9,10
$10,000
2,5,7
$15,000
6
Security
• Assigning privileges to different users
GRANT SELECT on Customer TO Jones
GRANT INSERT ON Part TO Smith, Park
Integrity Rules
• Entity Integrity
– No field that is part of a primary key can be
NULL.
– NULLs are when a value is missing, unknown
or inapplicable
Referential Integrity
• Relationships are not very obvious.
• Picture a DB with 20 tables, where tables have
30 fields.
• Foreign Key
FOREIGN KEY (RepNum) REFERENCES Rep
In the customer table
If they don’t match (perhaps a rep has been deleted)- then
referential integrity is compromised.
Structure Changes
ALTER TABLE Customer ADD CustType CHAR(1)
ALTER TABLE Customer
CHANGE COLUMN CustomerName TO CHAR(40)
ALTER TABLE Part
DELETE Warehouse
DROP TABLE SmallCust
Many to Many Relationships
• Students to Teachers
• Products to Categories
• Musicians to Bands
Need 3 Tables
Musicians (musicianID, name, …)
Bands (bandID, name, label…)
Musicians_bands (musicianID, bandID)