The Database
Database A shared collection of logically related data, and a description of this
data, designed to meet the information needs of an organization.
We now examine the definition of a database to understand the concept fully. The database
is a single, possibly large repository of data that can be used simultaneously by many
departments and users. Instead of disconnected files with redundant data, all data items are
integrated with a minimum amount of duplication. The database is no longer owned by one
department but is a shared corporate resource. The database holds not only the organization’s
operational data but also a description of this data. For this reason, a database is also
defined as a self-describing collection of integrated records. The description of the data is
known as the system catalog (or data dictionary or metadata – the ‘data about data’). It
is the self-describing nature of a database that provides program–data independence.
The approach taken with database systems, where the definition of data is separated
from the application programs, is similar to the approach taken in modern software
development, where an internal definition of an object and a separate external definition
are provided. The users of an object see only the external definition and are unaware of
how the object is defined and how it functions. One advantage of this approach, known as
data abstraction, is that we can change the internal definition of an object without affecting
the users of the object, provided the external definition remains the same. In the same
way, the database approach separates the structure of the data from the application programs
and stores it in the database. If new data structures are added or existing structures
are modified then the application programs are unaffected, provided they do not directly
depend upon what has been modified. For example, if we add a new field to a record or
create a new file, existing applications are unaffected. However, if we remove a field from
a file that an application program uses, then that application program is affected by this
change and must be modified accordingly.
The final term in the definition of a database that we should explain is ‘logically related’.
When we analyze the information needs of an organization, we attempt to identify entities,
attributes, and relationships. An entity is a distinct object (a person, place, thing, concept,
or event) in the organization that is to be represented in the database. An attribute is a
property that describes some aspect of the object that we wish to record, and a relationship
is an association between entities. For example, Figure 1.6 shows an Entity–
Relationship (ER) diagram for part of the DreamHome case study. It consists of:
n six entities (the rectangles): Branch, Staff, PropertyForRent, Client, PrivateOwner, and Lease;
n seven relationships (the names adjacent to the lines): Has, Offers, Oversees, Views, Owns,
LeasedBy, and Holds;
n six attributes, one for each entity: branchNo, staffNo, propertyNo, clientNo, ownerNo, and leaseNo.
The database represents the entities, the attributes, and the logical relationships between
the entities. In other words, the database holds data that is logically related. We discuss the
Entity–Relationship model in detail in Chapters 11 and 12.
Database A shared collection of logically related data, and a description of this
data, designed to meet the information needs of an organization.
We now examine the definition of a database to understand the concept fully. The database
is a single, possibly large repository of data that can be used simultaneously by many
departments and users. Instead of disconnected files with redundant data, all data items are
integrated with a minimum amount of duplication. The database is no longer owned by one
department but is a shared corporate resource. The database holds not only the organization’s
operational data but also a description of this data. For this reason, a database is also
defined as a self-describing collection of integrated records. The description of the data is
known as the system catalog (or data dictionary or metadata – the ‘data about data’). It
is the self-describing nature of a database that provides program–data independence.
The approach taken with database systems, where the definition of data is separated
from the application programs, is similar to the approach taken in modern software
development, where an internal definition of an object and a separate external definition
are provided. The users of an object see only the external definition and are unaware of
how the object is defined and how it functions. One advantage of this approach, known as
data abstraction, is that we can change the internal definition of an object without affecting
the users of the object, provided the external definition remains the same. In the same
way, the database approach separates the structure of the data from the application programs
and stores it in the database. If new data structures are added or existing structures
are modified then the application programs are unaffected, provided they do not directly
depend upon what has been modified. For example, if we add a new field to a record or
create a new file, existing applications are unaffected. However, if we remove a field from
a file that an application program uses, then that application program is affected by this
change and must be modified accordingly.
The final term in the definition of a database that we should explain is ‘logically related’.
When we analyze the information needs of an organization, we attempt to identify entities,
attributes, and relationships. An entity is a distinct object (a person, place, thing, concept,
or event) in the organization that is to be represented in the database. An attribute is a
property that describes some aspect of the object that we wish to record, and a relationship
is an association between entities. For example, Figure 1.6 shows an Entity–
Relationship (ER) diagram for part of the DreamHome case study. It consists of:
n six entities (the rectangles): Branch, Staff, PropertyForRent, Client, PrivateOwner, and Lease;
n seven relationships (the names adjacent to the lines): Has, Offers, Oversees, Views, Owns,
LeasedBy, and Holds;
n six attributes, one for each entity: branchNo, staffNo, propertyNo, clientNo, ownerNo, and leaseNo.
The database represents the entities, the attributes, and the logical relationships between
the entities. In other words, the database holds data that is logically related. We discuss the
Entity–Relationship model in detail in Chapters 11 and 12.
No comments: