Databases
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Shortfalls of file management systems
Structure of a database
Database administration
Database Management system
Hierarchical Databases
Network Databases
Relational Databases
SQL
Desirable Features in an
Information System
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Integrity
Data independence
Controlled redundancy
Security
Privacy
Database Definition
• A computerised record-keeping system.
• Used by a range of users who have different
requirements.
– Minimal enquiries.
– In-depth updating.
– Restructuring.
• A well-implemented database will have data
integrity, data independence, controlled
redundancy, security and privacy, for all users.
Uses of a Database
• Generally used for on-line transaction
processing (OLTP)
• Data warehouses are a hybrid of databases
which are used for on-line analytical
processing (OLAP)
Structure of a database
External
Schema
Conceptual
Schema
Internal
Schema
Physical
Schema
External Level
• Level visible to user
• Multiple views of the system
– e.g. View an order - see limited product and
customer information
• Only the database Administrator may access
the whole database at this level
EXTERNAL SCHEMA
• Each external view is defined by means of an
external schema
• Provides definitions of each external view.
• Written in a Data Definition Language.
• individual to the user or user group.
• accessed through a 3GL, a query language or a
special purpose forms or menu-based language
Conceptual level
• CONCEPTUAL - represents the entire
information content of the database
• Consists of multiple types of conceptual record.
This level preserves the data independence of the
database.
• CONCEPTUAL SCHEMA - defines each of the
various types of conceptual record, in a conceptual
Data Definition Language.
Internal level
• INTERNAL - a low-level representation of the
entire database; it consists of multiple occurrences
of multiple types of internal record. It is the stored
record, inasmuch as it contains all but the devicespecific information on the storage of the
database.
• PHYSICAL - the physical device and block
addresses for each of the records.
Mappings
• Each level maps onto adjoining levels
• conceptual / internal mapping specifies how
conceptual records and fields are represented at
the internal level
• Changes can be made in the internal level without
affecting the conceptual level
• external / conceptual mapping defines the
correspondence between an external view and the
conceptual view
DBMS - Database Management
System
• software handling access to the database
• allows both the database administrator and
all users the access to the database to which
they are entitled
How requests are processed
• User issues request (e.g. through SQL)
• DBMS intercepts and analyses request
• DBMS inspects user's external schema, external to
conceptual mapping, conceptual schema,
conceptual to internal mapping and the storage
structure definition.
• DBMS executes operations on stored database.
DATABASE ADMINISTRATOR
(DBA)
• Decide on the storage structure and access
strategy
• Liase with the users
• Define security and integrity checks
• Define a backup and recovery strategy
• Monitor and respond to performance
Utilities used by the DBA
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Load routines
Dump/Restore routines
Reorganisation routines
Statistics routines
Analysis routines
Data dictionary (containing METADATA, which
gives data descriptions and mappings)
Database models
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Hierarchical
Network
Relational
Object-oriented
Hybrid
Data Warehouses
Relational database
• Data is independent from programs and from other
data
• Data is represented in TABLES rather than files.
(one entity corresponds to 1 table)
• Column headings are described as DOMAINS.
(i.e. attributes)
• Items of information as TUPLES or ROWS rather
than records (i.e. occurrences of the entity)
Definitions
• A RELATION is a collection of semantically related
information, usually containing a unique key. A
RELATION = a Table
• FOREIGN key - a key to a different relation that is used as
non-key data in this relation. (i.e. the enforcing field in the
relationship)
• SIMPLE key - uses one item from the row
• COMPOUND key - uses more than one item / attribute
• Unnormalized data - contains headings, footings, differing
number of occurrences for different fields.
Properties of a relation (TNF
test)
• All row entries are non-divisible (atomic) i.e. no such thing as arrays
• All entries in a particular column are drawn
from the same set (i.e. no such thing as
redefines)
Normalisation of data
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Collect all documents to be entered/produced
Represent documents in unnormalized form
Choose and identify key items, giving unnormalized data + keys
Separate out repeating groups -> 1st Normal Form (1NF)
Separate out part key dependencies -> 2nd Normal Form (2NF)
Separate out inter-data and inter-key dependencies -> 3rd
Normal Form (TNF)
• Apply TNF tests
• Optimise by combining relations with identical keys
• Apply TNF tests again
Relational database
• This is a database that is perceived by its users as a
collection of tables. Each table can define an ENTITY
• Entities can be related through RELATIONSHIPS
• Relationships are implemented by use of foreign keys in
tables
• Each column has a unique name within the table
• All rows are distinct (no two are the same)
• Row or column order is not significant
• Every relation must have a key
Implementation of desirable
features
• Integrity
– A field’s validation can be declared when the field is
declared. If this validation is used, then the integrity of
the field remains intact.
– Entity integrity - No attribute participating in the
primary key of a base relation is allowed to accept null
values.
– Domain constraints - what are the possible valid values
that can be used?
Referential integrity
– Through the propagation and use of foreign
keys, no detail can be created where a master is
needed, nor can a master be deleted without
consent to the deletion of the details
Implementation of desirable
features
• Data independence
– The implementation of relational databases
causes the external and conceptual schema to
be data independent. The internal schema and
the physical level are data dependent.
• Controlled redundancy
– The relational model reduces redundancy at the
conceptual level
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