Sunday, February 17, 2013

Data dictionary

­­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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