OVERVIEW OF MAJOR COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 58

dictionary refers to the storage of structure definitions—the storage repository of
structure definitions. Apart from this storage repository itself, within a DBMS you
have software components to link the data dictionary repository with other soft-
ware components. That data dictionary interface software is considered a distinct
component within a DBMS.
Let us now examine all aspects of data dictionary—both the storage of structure
definitions and the software interface to the data dictionary storage.
Consider a distinct difference between database systems and file-oriented
systems. The difference lies in the way data is accessed. Database systems enable
you to access physical data in a more abstract manner. You do not have to specify
the physical layout of the data structure of how data is stored, nor do you need to
indicate where exactly the data is to be found on secondary storage media. This
provides you with enormous flexibility and ease in writing programs with database
operations. The data dictionary is a fundamental component providing this capa-
bility. In most relational databases, the data dictionary comprises a set of system
tables. It is a like a directory system. The data dictionary is also known as meta-
data—data about data.
Particularly, the data dictionary performs the following tasks:
• Keeps the definitions of all data structures and relationships
• Describes data types of all stored data items
• Specifies field sizes and formats of data items
• Holds information on space allocations and storage locations for all data
• Enables the database engine to keep track of the data and store it in the
assigned places
• Maintains indexes
• Stores report and screen definitions
Query Processor
Power users, who know more about computer capabilities and usage, write queries
to retrieve information from the enterprise database. You, as a programmer or
analyst working within the Information Technology department, formulate and
write queries to be used by other user groups. Specialized users like business
analysts, engineers, and scientists also write their own queries for their information
requirements.
To write queries, you need a language to communicate your request to the com-
puter system. Each commercial vendor adopts a standard query language or devel-
ops its own proprietary language for creating queries. SQL (Structured Query
Language) has become the standard for relational DBMSs. Each relational data-
base vendor enhances the standard SQL and adds a few features here and there,
but the essential features of SQL remain in every commercial DBMS. You write
queries with SQL and run them in your database environment.
Let us say you write the following query in SQL for the marketing VP in your
company, who wants to plan a marketing campaign for promotional mailings to a
certain segment of the customer base: