08 THE DATA DICTIONARY Structures & Elements

1. And Franchise Colleges 08 THE DATA DICTIONARY Structures & Elements • By MANSHA NAWAZ Data Dictionary - Structures & Elements

2. Introduction • We've seen how to build up a useful model of a system using DFDs. • The technique of explosion within DFDs is a powerful way of breaking a complexproblem down into a number of simpler problems. • However, we still haven't reached the level of detail that would be needed by a developer to actually build the system. • This is provided by the Data Dictionary. • A Data Dictionary is a collection of further details about everything on a set of DFDs, not only about data, but also about processes. Data Dictionary - Structures & Elements

3. Recording Data in the Data Dictionary • The data parts of DFDs are data flows and data stores. • On the DFD we have just a name for these, so we need to record the content and structure of these before we can develop software. • For our logical model, we want a way of describing the data that will accommodate any physical implementation. • This is done by describing the data in terms of data elements and data structures. Data Dictionary - Structures & Elements

4. The Data Dictionary • it is a receptacle for storing information on all system items • data stores • data flows • processes • external entities • it contains a logical description Data Dictionary - Structures & Elements

5. Document : Design Specification • Data Dictionary • Data Stores and Flow lecture 08 • Structures & Elements • Data Stores and Flow • Usage lecture 09 • Process Descriptions • Mini Specs and Sample Screen Shotslecture 10 • NORMALISATION lecture 11 • Database Tables derived from Data Store Descriptions Data Dictionary - Structures & Elements

6. SYSTEMS DESIGN • THE DATA DICTIONARY • : Data Structure & Elements Data Dictionary - Structures & Elements

7. Learning Aims • Aim • to be able to describe data in terms of its constituent structures and elements • Objectives • to identify indivisible data items • to record elements using a given syntax • to identify collections of data items • to record structures using a given syntax Data Dictionary - Structures & Elements

8. Lecture Outline • the need to describe data • a data hierarchy • data elements • data structures • Yourdon’s data notation Data Dictionary - Structures & Elements

9. Describing Data • the DFD gives a logical view of a system’s functionality • a DFD provides use with DATA STORES & DATA FLOWS • the designers/programmers will need to know the logical composition of the data • what it is and not the actual values Data Dictionary - Structures & Elements

10. Starting Point: • Each DATA STORE requires a data description comprising of its Data Structures & elements • Each DATA FLOW may requires a data description comprising of its Data Structures & elements (only record for dataflow whose data description is different from its datastore) Data Dictionary - Structures & Elements

11. A Data Hierarchy • data seen as a two level hierarchy • data elements • smallest logical item • data structures • collection of elements • collection of structures • collection of both Data Structure Data Element Data Dictionary - Structures & Elements

12. Data Elements • the items of data within the problem domain that can’t be further divided • for example an address • 15 High St, M’boro, Cleveland , TS4 5KL • this can be broken down into elements • a house number, a road, a town, a county and a postcode • each of these can’t be further divided Data Dictionary - Structures & Elements

13. Data Structures • the items of data that can be further sub-divided • the data item ‘address’ is a structure because it can be further divided into meaningful data items • here the structure consists of a number of elements but it is possible to have structures nested within structures Data Dictionary - Structures & Elements

14. Example • Structures: ADDRESS = houseno + road + town + county + postcode • Elements: houseno = 1{char}3 **usually numeric road = 1{char}15 town = 1{char}10 county = 1{char}10 postcode = 6{char}7 ** format $$# #$$ where each $ = [A..Z] each # = [0..9] often the elements are described using the types found in databases - char, integer, real, boolean, date Data Dictionary - Structures & Elements

15. Yourdon’s Notation X = Y X is composed of Y X + Y X and Y (X) X is optional item j{X}k iterated item, j/k low/upper limit [X | Y] item to be selected from X or Y @X identifier item ** comment Data Dictionary - Structures & Elements

16. Avoid Redundancy • this division of data in elements and structures allows analysts to re-use defined items • other structures can have the structure ADDRESS as a constituent part but the elements that make up ADDRESS and ADDRESS itself need only be defined once Data Dictionary - Structures & Elements

17. Recording Elements & Structures • check each data item in the system • identify all elements & structures • create a list of each type and record • name • elements lower , STRUCTURES upper case • description • one sentence english text • composition • use given notation Data Dictionary - Structures & Elements

18. Contd. • aliases • other names used, delivery note/despatch note • discrete values or continuous range limits • NZ = New Zealand or 100-1000 • default value • preset value on creation • validation information • if title = Mr then sex = Male • related data items • houseno, associated with structure ADDRESS Data Dictionary - Structures & Elements

19. Document : Data Dictionary • Data Dictionary lecture 08 • Data Stores - Structures & Elements lecture 08 • Start by listing each DATA STORES • For each data store list structures and elements • Draft a form view for the data store • Data Flow - Structures & Elements lecture 08 • Start by listing each DATA FLOW • Copy and paste the related data store structures and elements • Cut or delete the structures and elements which are not required • Draft a form view for the data flow. • Data Stores and Flow Usage lecture 09 • Process Descriptions lecture 10 Data Dictionary - Structures & Elements

20. Please Note: • Ascent allows you to record the Data Dictionary Structures and Elements and a guide has been produced DD in Ascent. • Unfortunately Ascent is weak when producing data dictionary content for a report. So feel free to utilize MS Word when creating your Data Dictionary Structures and Elements. Data Dictionary - Structures & Elements

21. Summary For each Data Store describe logical data data elements data structures notation minimal redundancy Self Study review the lecture complete exercises from the course workbook Summary & Self Study Data Dictionary - Structures & Elements

22. Further Reading • Notes from Annette Marshall • http://outranet.scm.tees.ac.uk/users/u0018257/SDV/SDevWk5.doc Data Dictionary - Structures & Elements