Database systems

Diomidis Spinellis
Department of Management Science and Technology
Athens University of Economics and Business
Athens, Greece
dds@aueb.gr

Advantages

Data independence
Efficient data access
Data integrity
Data security
Data administration capabilities
Resilient concurrent access
Crash recovery
Reduced application development time

Problems and Limitations

Data Management approach poses problems in data resource management
Developing a large database and installing a DBMS can be difficult and expensive
More hardware capability is required (storage and memory requirements are higher)
Longer processing times may result from high-volume transaction processing applications
The security and integrity of an organization's databases are major concerns of an organization's data resource management effort

Levels of Abstraction

Physical schema: Defines how data is stored
Conceptual schema or logical schema: Defines data in terms of a data model
External schema or view level: Defines a number of simplified domain-specific views

DBMS Levels of Abstraction

Data Independence

The three different levels of abstraction allow us to:

Change the file organization without affecting the conceptual schema
Change the conceptual schema without affecting an application (by adjusting the external schema)

Database Design

Requirements analysis
Conceptual database design: develop high level description
Logical database design: map description into the specific DBMS
Schema refinement: identify and solve potential problems
Physical database design: optimize for specific workloads
Security design

Entities and Attributes

Entity: Object in the real world
Attributes: Describe each entity
Entity set: Group of similar entities (share same attributes)
Attribute domain: Values that can be used for the attribute
Key: Minimal set of attributes that uniquely identify an entity
Primary key: Designated among a number of candidate keys

The books entity set

The Entity Relationship Model

Relationship: Association between two entities
Relationship set: Group of similar relationships
Descriptive attributes: can identify a relationsip

A relationship can be identified as:

one-to-many
many-to-many
one-to-one

The Published relationship

The Relational Model

Relation Schema

Name of the relation
Name of each field (or column or attribute)
Domain of each field

Relation Instance

Set of tuples (or records)

(Table of rows with the same number of fields)

ISBN	Title	Price
0-387-02620-7	Beyond Fear	29.00
0-201-79940-5	Code Reading	49.95
0-07-246535-2	Database Management Systems	40.00
0-19-502402-8	The Timeless Way of Building	60.00

Redundancy

Storing redundant information in a database results in:

Waste of space
Data that may not be correctly updated
Data that can not be inserted unless other data is also inserted
Data that is incidentally lost when other data is removed

Example

ISBN	Title	Price	Discount	Reduced Price
0-387-02620-7	Beyond Fear	30.00	10	27.00
0-201-79940-5	Code Reading	50.00	20	40.00
0-07-246535-2	Database Management Systems	40.00	20	38.00
0-19-502402-8	The Timeless Way of Building	60.00	10	54.00

Dependencies are resolved by decomposing the relations.

The Structured Query Language

Language used for

Creating
Manipulating
Querying

relational DBMSs.

Used:

Directly by database administrators and expert users
As a standard way for applications to communicate with the DBMS
As a way to backup, restore, and transfer data in a portable form

SQL Table Definition and Data Manipulation

Create the Table

CREATE TABLE Books (
        isbn CHAR(13),
        title CHAR(100),
        price real)

Add a Record

INSERT
INTO Books (isbn, title, price)
VALUES ("0-201-79940-5", "Code Reading", 49.95)

Modify Records

UPDATE Books B
SET B.price = B.price * 1.1

Delete a Record

DELETE
FROM Books B
WHERE ISBN="0-201-79940-5"

SQL Queries

Display the Details for a Record

SELECT *
FROM Books
WHERE ISBN="0-201-79940-5"

Display Records Based on a Condition

SELECT title, price
FROM Books
WHERE price > 50 AND price > 10

Perform an Aggregate Calculation

SELECT AVG(price)
FROM Books

Join Two Tables

SELECT title, authorname
FROM Books, Authors
WHERE Books.authorid = Authors.authorid

Query By Example

SELECT BOOKS.title, BOOKS.price, BOOKS.price, BOOKS.isbn
FROM BOOKS
WHERE (((BOOKS.price)>100)) OR (((BOOKS.price)<10))
ORDER BY BOOKS.title;

Additional SQL Query Constructs

SQL provides many sophisticated capabilities:

Selection of distinct records
SELECT DISTINCT title, price
FROM Books
WHERE price < 50 AND price > 10
Aggregate calculations (COUNT, SUM, AVG, MIN, MAX)
SELECT MIN(price), MAX(price)
FROM Books
Operations on groups
SELECT discount, AVG(price)
FROM Books
WHERE price > 10
GROUP BY discount
HAVING MAX(price) - MIN(price) > 10
String pattern matching
SELECT title, price
FROM Books
WHERE title LIKE 'TIMELESS%'
Ordering
SELECT title, price
FROM Books
ORDER BY price
Set union, intersection and difference (UNION, INTERSECT, EXCEPT)
SELECT title, price
FROM Books
UNION
SELECT title, price
FROM magazines
Nested queries (IN, EXISTS, operator ANY, operator ALL)
SELECT title, price
FROM Books
WHERE price > (SELECT AVG(price) FROM Books)

SELECT title, price
FROM Books
WHERE price < ALL
        (SELECT price
         FROM Books
         WHERE title LIKE 'Harry Potter%')

Views

By defining appropriate views on a schema we

Achieve logical data independance
Can better organise security

CREATE VIEW BrowseBooks(isbn, title)
        AS SELECT B.isbn, B.title
        FROM Books B
        WHERE B.price < 100

Indexes

An index is an auxilliary data structure used to speed up operations that are not efficiently supported by a table's record organisation.

CREATE INDEX IndPrice ON Books
WITH STRUCTURE = BTREE
KEY = (price)

To design the index structure take into account:

The query workload
The update (insert, delete, update) workload
Operations that will benefit from the index
Attributes that are often used
The type of the index structure (hash versus tree)
Index maintenance cost

Security Issues

Our goal in designing a secure database is to achieve:

Confidentiality
Integrity
Availability

Discretionary access control provides us the capability to give (and revoke) rights to specific users or groups.

Examples

GRANT SELECT
ON BrowseBooks
TO WebUsers

REVOKE INSERT, DELETE
ON Books
From Alice

GRANT INSERT, DELETE
ON Books
TO InventoryGroup

GRANT UPDATE(price)
ON Books
TO MarketingGroup

GRANT UPDATE(title, isbn)
ON Books
TO MaintenanceGroup

Transaction Management

A transaction is a single unit of execution program execution (e.g. book a seat)
Transactions provide concurrency control
Transactions support crash recovery

Four important properties (ACID):

Atomic: Database ensures that all actions are carried out, or none
Consistency: Users ensure that transactions leave the data in a consistent state
Isolation: Users to not have to worry about concurrently executing transactions
Durability: Completed transactions persist after a crash even if the database has not been updated

Crash Recovery

Crash recovery mechanisms guard against
- System crashes
- Media failures
A log or journal records all changes before they modify the database
The log is assumed to survive system crashes and media failures
After a crash the recovery manager follows the ARIES stretegy

Analysis

of changes that have not been written and active transactions

Redo

actions that were not written to the database

Undo

transactions that were not completed
The log is also updated during the recovery to guard against repeated crashes
A checkpoint is periodically recorded to the log to reduce the recovery overhead
A checkpointing strategy is also used to take backup copies of live databases

The Database Administrator

Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise's information resources and needs.

The database administrator's duties include:

Schema definition
Storage structure and access method definition
Schema and physical organization modification
Granting user authority to access the database
Specifying integrity constraints
Acting as liaison with users
Monitoring performance and responding to changes in requirements

User Profiles

Users are differentiated by the way they expect to interact with the system

Application programmers: interact with system through DML calls
Sophisticated users: form requests in a database query language
Specialized users: write specialized database applications that do not fit into the traditional data processing framework
Naive users: invoke one of the permanent application programs that have been written previously

RAID Storage

Redundant arrays of independent disks (RAID) are used to overcome two bottlenecks associated with disk storage:

Performance
Reliability

By using additional redundant disks we can increase both. The following are some typically identified RAID levels:

0: Nonredundant: Data is distributed across different disks to increase performance
1: Mirrored: A second disk set keeps a copy of the data (50% overhead)
0+1 (or 10): Striping and mirroring: Data is distributed across a second disk set to increase performance
2: Error correcting codes: Additional check disks (4:3, 10:4, 25:5) are used to provide redundancy with a smaller overhead (e.g. 57%, 71%, 83%)
3: Bit-interleaved parity: A single additional check disk is used to recover the data by distributing bits across all disks
4: block-interleaved parity: The check disk contains the parity on a block level: higher read throughput
5: block-interleaved distributed parity: Block parity is distributed across all disks: higher read and write throughput
6: higher redundancy: Like 5 with an additional check disk guarding against a second failure

XML Concepts

Semantic and structural text markup language
Does not (directly) define presentation
Application of SGML derived from HTML
Extensible
A separate document type definition is used to specify the structure of the data
Delivers cross-platform data portability
Domain-specific applications allow data interchange

XML Syntax

An XML document is structured on the following building blocks:

Elements

Delimited by starting and ending tags
Example:

<title>The Lord of the Rings</title>

Elements can also be empty; a special shortcut makes them smaller:
Example:

is the same as

<dvd />

Attributes

Name=valua pairs in the starting tag, that can be used for metadata
Example:

<title lang="english">The Lord of the Rings</title>

Entity references

Are used to refer to special (reserved) symbols

Symbol	Entity reference
<	<
>	>
&	&
"	"
'	'

Comments

Delimited by the <!-- and -> sequence
Example:

<!-- this is a comment ->

Example

<booklist>
        <book>
                <isbn>0-387-02620-7</isbn>
                <title>Beyond Fear</title>
                <author>
                        <givenname>Bruce</givenname>
                        <familyname>Schneier</familyname>
                </author>
                <price>30.00</price>
        </book>
        <book>
                <isbn>0-201-79940-5</isbn>
                <title>Code Reading</title>
                <author>
                        <givenname>Diomidis</givenname>
                        <familyname>Spinellis</familyname>
                </author>
                <price>50.00</price>
        </book>
        <book>
                <isbn>0-07-246535-2</isbn>
                <title>Database Management Systems</title>
                <author>
                        <givenname>Raghu</givenname>
                        <familyname>Ramakrishnan</familyname>
                </author>
                <author>
                        <givenname>Johannes</givenname>
                        <familyname>Gehrke</familyname>
                </author>
                <price>40.00</price>
        </book>
        <book>
                <isbn>0-19-502402-8</isbn>
                <title>The Timeless Way of Building</title>
                <author>
                        <givenname>Cristopher</givenname>
                        <familyname>Alexander</familyname>
                </author>
                <price>60.00</price>
        </book>
        <book>
                <isbn>3-8218-0479-3</isbn>
                <title lang="german">Lexikon der Populaeren Irrtuemer</title>
                <author>
                        <givenname>Walter</givenname>
                        <familyname>Kraemer</familyname>
                </author>
                <author>
                        <givenname>Goetz</givenname>
                        <familyname>Trenkler</familyname>
                </author>
                <price>40.00</price>
        </book>
</booklist>

Document Type Definitions

A document type definition is used to describe the format of valid XML documents
It can be used to verify XML documents
It can be used to parse XML documents decomposing them into a tree
Defines allowed elements
For each element defines content and supported attributes
Specified element content can be

Parsed character data

Indicated by #PCDATA

Child element

Indicated (name)

Sequence of elements

Indicated by (child1, child2, ...)

Zero or one child elements

Indicated by name?

Zero or more child elements

Indicated by name*

One or more child elements

Indicated by name+

A choice among different child elements

Indicated by (name1 | name2 | ...)

Example:

  <!ELEMENT booklist (book)*>
    <!ELEMENT book (isbn,title,author+,price,edition?)*>
      <!ELEMENT isbn (#PCDATA)>
      <!ELEMENT title (#PCDATA)>
      <!ATTLIST title lang (german|english) "german">
      <!ELEMENT author (givenname,familyname)>
        <!ELEMENT givenname (#PCDATA)*>
        <!ELEMENT familyname (#PCDATA)>
      <!ELEMENT price (#PCDATA)>
      <!ELEMENT edition (#PCDATA)>

Bibliography

D. D. Chamberlin, M. M. Astrahan, K. P. Eswaran, P. P. Griffiths, R. A. Lorie, J. W. Mehl, P. Reisner, and B. W. Wade. SEQUEL 2: A unified approach to data definition manipulation and control. IBM Journal of Research and Development, 20(6):560-575, November 1976.
Peter Pin-Shan Chen. The entity-relationship model --- toward a unified view of data. ACM Transactions on Database Systems, 1(1):9-36, March 1976.
Elliotte Rusty Harold and W. Scott Means. XML in a Nutshell. O'Reilly and Associates, Sebastopol, CA, 2001.
International Organization for Standardization, Geneva, Switzerland. Information technology --- Database languages --- SQL, 1992. ISO/IEC 9075:1992.
Henry F. Korth and Abraham Silberschatz. Database System Concepts. McGraw-Hill, second edition, 1991.
Raghu Ramakrishnan and Johannes Gehrke. Database Management Systems. McGraw-Hill, second edition, 2000.

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