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Entity relationship diagrams

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Data modeling using ER Entity relationship diagrams Data modeling using ER * Data modeling using ER * Why analyze data? Insight in data - and structures of data. – PowerPoint PPT presentation

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Title: Entity relationship diagrams


1
Data modeling using ER
  • Entity relationship diagrams

2
Why analyze data?
  • Insight in data - and structures of data.
  • Create a structure in unstructured data
  • Prior to designing a database
  • A part of the systems development process.

3
ER modeling
  • ER modeling is a widely used technique for data
    analysis.
  • Many notations (syntax) for ER diagrams
  • Different book ? different notations
  • Diagrams can be drawn using Visio or a similar
    tool.

4
Entities with attributes
  • Characteristics of an entity
  • An important thing in the world we model.
  • Entities have attributes (data)
  • Similar to a class in OOA, but has no methods /
    functions.
  • Notation (this book)
  • Entity Rectangle
  • Attribute Oval

5
Types of attributes
  • Simple or composite attributes
  • simple (atomic) e.g. age
  • Notation Oval
  • composite
  • address street number ZIP code town
  • Notation Oval connected to several sub ovals.

6
Types of attributes II
  • Single valued or multi-valued
  • Single valued e.g. CPR
  • Multi-valued e.g. phone number
  • One person may have several phones
  • Notation Double oval
  • An attribute may be composite and multi-valued!

7
Types of attributes III
  • Derived Calculated from other attributes e.g.
    sum or average
  • Notation Oval with a dashed border
  • An attribute with no value has value NULL
  • Values does not exist.
  • Value unknown.

8
Entities
  • An entity type is like a class
  • Naming convention Substantive, singular
  • Every instance of the entity must be unique
  • Identified by a key, one or more attributes
  • Notation for keys Underline the name of the
    attribute(s)

9
Relationships
  • Relationships between entities.
  • Notation Rhomb
  • Similar to association in OOA.
  • Relationships may have attributes
  • Notation Oval

10
Relationships II
  • Binary relationship
  • Relationship between 2 entities (normal)
  • Relationships, higher degree
  • Relationship between more entities (rare)
  • Recursive relationships
  • Relationship of 2 entities belonging to the same
    entity type e.g. an employee with a relation to
    another to a boss (another employee)

11
Cardinality
  • 1-1 very rare
  • 1 entity of type A has a relation to at most 1
    entity of type B and vice versa
  • 1 husband 1 wife
  • 1-many, 1-N very common
  • 1 entity of type A may have relation to many
    entities of type B, not vice versa.
  • 1 department many employees

12
Cardinality II
  • Many - many, N-M common
  • 1 relation of type A may have relations to many
    entities of type B and vice versa.
  • 1 employee many projects
  • 1 project many employees

13
Cardinality notation
  • Step into the entity type and look at the other
    entity through the relation.
  • How many entities of the other type do you see?
  • Write the number on far end of the relation.

14
(min, max) cardinality
  • Often youre only concerned with max.
    cardinalities.
  • Sometimes you want both (min, max) cardinality.
  • Note Cardinalities placed in the other end of
    the relation!!!

15
History
  • History More generations of relations
  • Cardinality goes up
  • 1 man many wives not simultaneously
  • Put a timestamp / period on the relation
    (attribute on the relation)
  • Relation marriage start_date, end_date
  • end_date may be NULL -)

16
Total participation
  • Ordinary relation
  • An entity may be related to another entity.
  • Total participation
  • En entity must be related to another entity.
  • Min. cardinality 1 (not 0)
  • Notation Double line on the relation

17
Weak entities
  • An entity that is dependent on some other entity.
  • If the strong entity is deleted so is the weak
    entity.
  • Notation
  • Weak entity Rectangle with double borders
  • Identifying relationship Rhomb with double
    borders (shows the strong entity)

18
Weak entities II
  • A weak entity has a partial key
  • Not unique
  • Unique in combination with the key from the
    strong entity.
  • Notation Dashed line under the partial key.

19
Diagramming tools
  • Drawing
  • Microsoft Visio now a part of MS Office
  • Diagramming to generate tables
  • You draw ER, the tool generates tables
  • Reverse engineering
  • You have the tables, the tools generates ER.
  • Dont use tools early in the process - use pen
    and paper!!!

20
Best practice
  • Procedure (best practice)
  • find entities and easy attributes
  • relationships
  • cardinalities
  • keys
  • weak entities
  • total participation
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