Databases Reference
In-Depth Information
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Data repository
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Data dictionary
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Database software
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Data abstraction
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Data access
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Transaction support
We take a little time to review each of these in the following sections.
Data Repository
All data in the database resides in a
data repository.
This is the data storage unit
where the physical data files are kept and the central storage location for the data
content. The physical structure of the data repository is database specific.
Data Dictionary
The data repository contains the actual data. Let's say that you want to keep data
about your customers. There are two aspects of this data. One aspect is the struc-
ture of the data consisting of the field names, field sizes, data types, and so on.
The other part is the actual data values stored in the fields.
This first part, relating to the structure, typically is stored separately from
the database values as the data dictionary or data catalog. A
data dictionary
(data catalog)
contains the data element structures and the relationships among
data elements. The data dictionary and the data repository work together to pro-
vide information to users.
Database Software
Though sometimes referred to as databases, products like Microsoft Access, SQL
Server, MySQL, Oracle, Informix, and similar products are technically the software
that manages data and databases. These are
database software
or
database man-
agement systems (DBMS),
which support storing, retrieving, and updating data in
a database. The database software, or database management system (DBMS), is not
the database itself, but lets you store, manage, and protect the data in a database.
Data Abstraction
Consider a situation where you want to store information about customers. Data
about each customer consists of several fields such as customer name, street
address, city, state, zip code, credit status, and so on. Depending on users' require-
ments and roles in an organization, different users consider the data from differ-
ent levels of data abstraction.
Data abstraction,
in this context, is a way of look-
ing at data that breaks it down into its basic components and groups the
components to give you different ways of looking at the data. It gives you the abil-
ity to hide the complexities of data design at the levels where they are not required.
