Data Dictionary comprises two words i.e. data which simply means information being collected through some sources and dictionary means where this information is available.

Data Dictionary can be defined as collection of information of all data elements or contents of databases such as data types, text descriptions of system. It makes it easier for user and analyst to use data as well as understand and have common knowledge about inputs, outputs, components of a database, and intermediate calculations.

Why Data dictionary is essential ?

• There are less information and details provided by data models. So, a data dictionary is very essential and needed to have proper knowledge and usage of contents.
• Data Dictionary provides all information about names that are used in system models.
• Data Dictionary also provides information about entities, relationships, and attributes that are present in system model.
• As a part of structured analysis and design tool, implementation of a data dictionary is done.

Following type of Information is used to store in a data dictionary:

Name	Description
Name	Name generally includes primary name of all composite data or control item available, name of external entity or data store.
Aliases	Any other word used in place of Name
Where or How it’s used?	Data dictionary generally gives information about where and how data or control items are used which may include as an input/ output to process.
Description	A notation for representing content

Data Dictionary includes following types of notations:

Data Construct	Notation	Meaning
Composition	=	Is composed of
Sequence	+	Represents AND
Selection	[ |]	Represents OR
Repetition	{ }n	Represents n repetitions or repetition for n times
Parentheses	( )	Optional data that may or may not be present
Comment	*…*	Delimits a comment or commented information

Let us understand this by taking a example of reservation system. In reservation system, “passenger” is an data item whose information is available on data dictionary as follows:

Keys	Values
Name:	Passenger
Aliases	None
Where or how its used?	Passenger’s query (input) Ticket (output)
Description	Passenger = Passenger_name + Passenger_address Passenger_name = Passenger_lastname + Passenger_firstname + Passenger_middle_initial Passenger_address = Local_address + Community_address + Zip_code Local_address = House_number + street_name + Apartment_number Community_Address = City_name + State_name

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