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

Let’s examine the definition of a database in detail to understand this concept
fully. The database is a single, possibly large repository of data, which can be
used simultaneously by many departments and users. All data that is required
by these users is integrated with a minimum amount of duplication. And
importantly, the database is normally not owned by any one department or
user but is a shared corporate resource.
As well as holding the organization’s operational data, the database also
holds 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, that is
the meta-data – the ‘data about data’ – is known as the system catalog or data
dictionary. It is the self-describing nature of a database that provides what’s
known as data independence. This means that if new data structures are added
to the database or existing structures in the database are modified then the
application programs that use the database are unaffected, provided they don’t
directly depend upon what has been modified. For example, if we add a new
column to a record or create a new table, existing applications are unaffected.
However, if we remove a column from a table 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 organization’s information needs, we
attempt to identify the important objects that need to be represented in the
database and the logical relationships between these objects. The methodology
we’ll present for database design will give you guidelines for identifying these
important objects and their logical relationships.