Data Dictionary
It is store house of data, giving
information about data. During system implementation it serves as common base
for programmers who are working on the system, It compares their data
description. It is also an important step in building database.
“Data
dictionary contains description & definition consulting the data structure,
data elements, their interrelationship & other characteristics of a
system.”
Objectives of Data Dictionary
i.
A standard definition of all terms in a system i.e.
each data item is uniquely identified and defined.
ii.
Easy cross referencing between subsystem’s program and
modules.
iii.
Simple program maintenance.
iv.
It contains information about the data of the system
and there is an entry in the data dictionary for every element of DFD. Thus DFD
and Data Dictionary are compliment of each other.
Data Items:
There are three classes of data
items –
a.
Data
Element –
It is the
smallest unit of data which cannot be meaningfully decomposed further.
For
Ex:
Employee code, unit of measurement
b.
Data
Structure –
A group of
data elements forms a data structure.
For
Ex:
Data Structure of employee consist
of a group of data elements such employee code, name, age, experience, phone
no., address etc.
c.
Data Flow
& Data stores –
Data flows
are data structures in motion whereas Data stores are data structure at rest. Data
stores may be files, database etc.
Format of Data Dictionary:
A
data dictionary is organised into five sections –
i.
Data Elements
ii.
Data Flows
iii.
Data Store
iv.
Process
v.
External Entities
Data
Dictionary lists all data elements, flows, stores, process of the system and it
gives the detail about each item in following format –
Data
types, data name, data description, data characteristics, data control
information, composition, physical location of data, etc.
Data
dictionary for data element Employee code in dictionary –
Data
Element - Employee
code
Description - Unique
code assigned for each employee
Type - char
Length - 4
Range - 0-9999
Data
Stores - Employee table, Payroll table
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