HSQ - DATABASES & SQL

And Franchise Colleges HSQ - DATABASES & SQL 09 (a) ER Modelling In Practice DIGITAL VOICE SYSTEM ER Modelling In Practice

Case Study : Digital Voice System • This lecture introduces the Digital Voice Systems Scenario • The lecture uses a brief version of the scenario to introduce you to the model. • The full scenario and solution is on the website - you need to attempt the full scenario yourself. ER Modelling In Practice

Case Study : DIGITAL VOICE SYSTEM Modern private digital voice systems exchanges are basically complex computer systems. Private digital telephone networks are often used by organisations with a number of multi-building sites. These sites would be distributed around the country, each site would have one or more digital exchanges. At each site there would be a number of employees directly involved in the running of the voice systems. In particular a system would have to keep track of a telephone exchange manager for each site as well as the telephone operators. These digital exchanges are capable of providing a continual log of their activity. Every call is logged providing information as below: Outgoing calls: source extension, destination number, charge band. Incoming calls: destination extension, duration of call, number of rings before answered etc. Clearly details of many employees would need to be recorded together with the details of their associated telephone extensions. In practice employees might have more than one extension. Some employees also might share extensions and the system would have to be capable of coping with this. Organisations would divide their employees into departments. Calls would be charged against these department. It will be necessary that each extension would be associated with a single department for call charging. ER Modelling In Practice

Building Room Site m 1 1 1 m in in Exchange on m Modern private digital voice systems exchanges are basically complex computer systems. Private digital telephone networks are often used by organisations with a number of multi-building sites. These sites would be distributed around the country, each site would have one or more digital exchanges. • What entities can you see - a little imagination is required. • What about the relationships? Try to draw those in. • Site - Building - Room; a simple hierarchy. • You could also have a Floor entity - rooms are often named by building, floor, room number. ER Modelling In Practice

Building Building m Room Site Site m 1 1 1 1 1 m m m manages (voice) operates in in in Employee Exchange on 1 m At each site there would be a number of employees directly involved in the running of the voice systems. In particular a system would have to keep track of a telephone exchange manager for each site as well as the telephone operators. • Just use an Employee entity and model this with relationships. Requires a Table ER Modelling In Practice

Outgoing Call 1 1 m from to Extension m Incoming Call These digital exchanges are capable of providing a continual log of their activity. Every call is logged providing information As below: Outgoing calls: source extension, destination number, charge band. Incoming calls: destination extension, duration of call, number of rings before answered etc. • How do we model this bit? • You will need the Extension entity. (Telephone extension) • Try to add appropriate relationships. ER Modelling In Practice

Building Building m Room Site Site m 1 1 1 1 1 m m m manages (voice) operates in in in Employee Exchange on 1 m At each site there would be a number of employees directly involved in the running of the voice systems. In particular a system would have to keep track of a telephone exchange manager for each site as well as the telephone operators. • Just use an Employee entity and model this with relationships. Requires a Table ER Modelling In Practice

Outgoing Call 1 1 m from to Extension m Incoming Call These digital exchanges are capable of providing a continual log of their activity. Every call is logged providing information As below: Outgoing calls: source extension, destination number, charge band. Incoming calls: destination extension, duration of call, number of rings before answered etc. • How do we model this bit? • You will need the Extension entity. (Telephone extension) • Try to add appropriate relationships. ER Modelling In Practice

Building Building Outgoing Call m 1 operates m Site Site Room Requires a Table m 1 1 1 1 1 1 1 m m m manages (voice) in in from in Exchange Employee to on Plugged into 1 Extension m m m Incoming Call Reviewing what we have so far ... • Try to link these sections. • What is an extension connected to?. ER Modelling In Practice

m m m for 1 1 m m Extension Extension Employee Employee ?? Clearly details of many employees would need to be recorded together with the details of their associated telephone extensions. In practice employees might have more than one extension. Some employees also might share extensions and the system would have to be capable of coping with this. Starting from the Extension entity, how would you model this? Now decompose it. Now we need a name for the new entity, attributes and a primary key. Trying asymmetric viewing: For a particular extension - what is it a list of ? All the employees with that extension number. For an individual employee - what is this a list of? The employee’s extension number (or numbers) ER Modelling In Practice

on for 1 1 m m Extension Employee Naming the complex entity Directory Entry • Any ideas for a name for this entity? • How about Directory-Entry • Quite difficult to name…. • We need an attribute for Directory-Entry. • The primary key could be [extn#, emp#] • The entity records an employee being associated with an extension. Any ideas for attributes? • Would a date be appropriate? (always a useful guess) • The date the employee was given the extension number ER Modelling In Practice

Testing the primary key • The primary key could be [extn#, emp#] so we do the usual test. • Make up a few rows: • Now try to break the chosen primary key’ • Adding a row with the same primary key value as another row. • If we assume we only hold the latest date on which an extension is allocated to an employee - then the primary key seems to be appropriate. ER Modelling In Practice

Outgoing Call 1 1 1 1 m m from Charged to in to Department Extension Employee m m Incoming Call Organisations would divide their employees into departments. Calls would be charged against these department. It will be necessary that each extension would be associated with a single department for call charging. • Try to model this …. • To you to attempt (later) : • The full scenario is on the website. Try to re-model the scenario using the full scenario. • Try to derive the tables for the Digital Voice Systems model. ER Modelling In Practice

Outgoing Call 1 1 m from to Call Incoming Call Extension m call-type …….. Incoming Outgoing Internal Specialisation • Re-draw this as a specialisation structure • A better structure? • Specialisation provides a more powerful model for this type of situation. ER Modelling In Practice

End of Lecture ER Modelling In Practice

HSQ - DATABASES & SQL