1. Databases and DBMSs
Todd S. Bacastow
January 2005

2. A Process of Mapping Real World Conceptual Data Model Logical
Physical
Data Model
High level model
Comprises
“Things”
“Descriptions”
“How things are connected”
Relational
Hierarchical
Network
Object Oriented

3. Data Models A data model describes the structure of a database
data types, relationships, constraints
a set of basic operations
insert, delete, modify, retrieve
user-defined operations

4. Types of Data Models Conceptual Logical Physical
concepts: entity, attribute, relationship
Entity-Relationship model (DBMS-independent)
Logical
data represented by record structure
E.g. relational, network, hierarchical
Physical
describes how data is stored in the disk

5. DBMS Architecture External Level External View External View
Conceptual Level
Conceptual Schema
Internal Level
Internal Schema

6. Conceptual Schema
Internal Schema
External
View
External Level
Describes a part of the database for a particular user group and hides the rest
Supports multiple views of a database
Same data model as the conceptual schema
External Level
External
View

7. Conceptual Level Data Abstraction Conceptual Level
Conceptual Schema
Internal Schema
External
View
Conceptual Level
Data Abstraction
hides unnecessary details
Conceptual Level
hides physical layer
Data types, Constraints, User Operations
Uses both conceptual/logical data models
Conceptual Level
Conceptual Schema

8. Internal Level Defines physical storage on the disk
Conceptual Schema
Internal Schema
External
View
Internal Level
Defines physical storage on the disk
Defines data location
path, blocks, pages, …
Device specific
STORED_EMP BYTES=20
PREFIX BYTES=20, OFFSET=0
EMP# BYTES=20, OFFSET=6, INDEX=EMPX
DPET# BYTES=20, OFFSET=12
PAY BYTES=20, OFFSET=16, ALIGN=FULLWORD
Internal Level
Internal Schema

9. DB Schema vs. DB State Database Schema
description of the database
is specified during database design
Database State (extension of the schema)
current state of the database: a snapshot
actual data instances in a DB
changes over time by update
initially, a database is empty state with no data

10. DB Schema vs. DB State Valid State Schema Diagram
DBMS checks every state of the database
does it satisfy the structure and constraints specified in the schema?
Schema Diagram
Displays database schema

11. Example Schema

12. Designer Goal : develop a schema that changes infrequently
Database Schema
Metadata
descriptions of the schema constructs and constraints
stored in the database catalog
Schema Evolution
Schema change prompted by the change of application requirements

13. DBMS Mapping Mappings for multi-level DBMS Three-Schema Architecture
Conceptual Schema
Internal Schema
External
View
DBMS Mapping
Mappings for multi-level DBMS
to transform a request specified at one level into the request at another level
access: external  conceptual  internal  DB
retrieve: DB  internal  conceptual  external
Three-Schema Architecture
advantage: true data independence
disadvantage: overhead cost of mappings

14. Data Independence What happens when the schema changes at some level?
the capacity to change the schema at one level without having to change the schema at the next higher level
Two Types of Data Independence
logical and physical data independence

15. Data Independence (con’t)
1. Logical Data Independence
capacity to change the conceptual schema without having to change the external schema
when: logical reorganization of the database
2. Physical Data Independence
change the internal schema without having to change the conceptual schema
when: physical reorganization of the files

16. DBMS Languages Data Definition Language (DDL)
to define DB conceptual schema
Data Manipulation Language (DML)
to specify database requests: update, retrieval
high-level DML: describes which data to retrieve
low-level DML: describes how to retrieve it

17. DBMS Languages (con’t)
High-level DML: set-oriented, declarative
Low-level DML: record-oriented, procedural
Types of DML
data sublangauge: DML embedded in a general purpose language (for DBAs)
query language: high-level, interactive, stand-alone DML (casual end users)
user-friendly interface for DML (naïve users)

18. Database System Environment
DBMS Component Modules
Managers, i.e., disk control
Compiler, i.e., query
Processors

20. DBMS Interfaces Menu-based interfaces Forms-based interfaces
Natural language interfaces
interpret requests to high-level queries
Command line

21. System Utilities & Tools
Loading
loads existing data files into the database
DBMS conversion, reformatting the data
Backup
provides a backup copy of the database
incremental backup: updates changes only
File Reorganization
to improve performance

22. System Utilities & Tools
Performance Monitoring
monitors database usage
provides statistics
Data Dictionary
also called information repository
stores additional information: (catalog) + design decisions, usage standards, user information, application program descriptions

23. Mainframe/ terminal Storage, Logic and Presentation all in same place
Network
Mainframe/terminal
Storage, Logic and Presentation all in same place
No platform specific user interface
Doesn’t take advantage of client machine

24. Client Server without stored procedures
Storage
Logic
Presentation
Network
DBMS
Database server handles storage only
Logic and presentation in client
Takes advantage of client cpu
Logic changes require client redistribution
Integrity not maintained if other DB tool used
Each user needs to be a specific database user

25. Client Server with stored procedures
Storage
Logic
Presentation
Network
DBMS
Database handles storage and business logic
Logic changed in one place, no redistribution of client
DBMS dependent code
Each user needs to be specific database user

26. Client Server with 3 tiers
Storage
Logic
Presentation
Network
DBMS
Storage in database
Logic in Transaction Monitor
Client does presentation only
Authentication and Access control can be done in TP monitor
Each user does NOT have to be a database user

27. Client Server with 3 tiers
Database Servers
Storage
Logic
Presentation
Network
DBMS
Transaction
Monitor
A component which sits between the client and the database server to insure reliable updates of information
Used in airline reservation and banking systems

28. Why 3 Tiers? Scalability Flexibility Complexity
multiple transaction monitors
load balancing
Flexibility
Complexity
update multiple data stores
Two phase commit with multiple databases

29. Classifications of DBMSs
Data Model (OO, Relational, hierarchical)
Number of Users ( single vs. multi-user)
Number of Database Sites ( centralized vs. distributed vs. federated)
Special-purpose vs. general-purpose