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Data Modeling : ER Model…

Lecture 07

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Summary of last lecture•ER model notation•Examples•Ternary relationship•Weak entities

•Extended ER Model•Process Modeling

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EXTENDED E-R MODEL•extensions to capture more meaning•concepts of generalization, aggregation and sub-set hierarchies added• Similar to OO concepts : inheritance,

composite objects

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Generalization•to generalize from two or more entity sets and factor out commonality

•Example : given two entities Faculty and Non-faculty, we can define a ‘general’ entity called Employee

•Common attributes are factored out to define ‘Employee’ entity; specific (non-common) attributes incorporated in ‘Faculty’ and ‘Non-faculty’ entities

•represented by IS-A relationship

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Another example :

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Specialization•also called subset hierarchy; we created

special cases for a given entity. - Teacher a specific case of Employee - Employee called superset, Teacher called

subset•entity E1 is subset of E if every instance of E1

is also an instance of E; this is also IS-A relationship

•E called superset and E1 as subset (or sub-class); E may have multiple and possibly over-lapping subsets

•every instance in E need not be present in subsets of E

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Specialization…..•specialization allows classification of an entity in subsets based on some distinguishing attribute/property

•we may have several specialization of same entity

•the subsets may have additional attributes

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Inheritance•there is inheritance of attributes from superclass or superset

•the subclass/subset automatically inherits attributes defined at superclass/superset level

•thus, inheritance present in both Generalization and specialization• Direction important : bottom-up in

generalization, top-down in Specialization

• Important to distinguish the two cases

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Aggregation•for building complex entity from existing entities (or existing entities and relationships)

•two ways of defining complex entities : • create an attribute whose value is

another entity• define an entity as containing a group of

related entities

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Examples :•Work-order object (entity) defined as consisting of entities Raw-material, Tools and Workers;

•Work-order itself related with Customer entity

•Aggregation notation not explicitly provided in Extended E-R model

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Raw Material Tools

Worker

Customer

JobNo Quantity

Work-order

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Summary•Data Modeling important to understand

data to be handled in a given application.•We develop a conceptual Model first.•ER model uses concepts of Entity,

Relationship and Attribute.•Provides Constraints: primary key,

cardinality….•Simple to use diagramming notation•User cam understand and validate our

model

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Process Modeling

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Outline

•Process decomposition diagrams•Data flow diagram (DFD)

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Process Model•A process is a business activity which when executed produces certain outputs from given inputs

•The function(s) performed by a process may be complex, with multiple inputs, outputs and users

•The entire application itself is a process

•We use successive decomposition into sub processes to reveal greater details of the processing

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Function Decomposition•Decomposition splits wok of a task into subtasks; subtasks together make-up the parent task; not like ‘calling’ a module

•Balanced decomposition: sub-tasks are roughly equal in complexity

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Function Decomposition …•Top-down decomposition gives hierarchical structure

•Decompose into 2 or more; not more than 5

•A high cohesion (high independence) and minimum coupling (minimum interdependence) are fundamental criteria

•Continue decomposition until elementary processes are identified

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Function Decomposition•Elementary process is a smallest unit of activity meaningful to end user (it sees and leaves data in consistent state)

•Process decomposition diagram• A tree structure• Elementary processes are leaf nodes• Data are not shown

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FD Diagrams: Examples

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FD Diagrams: Examples

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•Exercise: prepare FDDs for• Railway reservation system• Hospital patient management• Employee payroll

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Function Decomposition …•Use proper naming of processes•Business functions named as nouns (marketing, Inventory control, …)

•Process name consists of an active verb and an object (accept order, calculate interest, …)

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Naming …•Avoid long names (sentences containing and, if, then, etc. indicate non-cohesive complex tasks)

•Real world is a good reference for selecting proper names; organizational units are organized functionally and each unit has a well-defined task