Data Modelling Advanced Features

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Data ModellingAdvanced Features (Special thanks
to Janet Francis for this presentation)
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Complex Relationships

• A university has lots of tutors and lots of
  modules
• A module is taught by many tutors
• A tutor teaches on many modules
• This leads to a Problem
• It is impossible to tell who teaches what!

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Many to Many Relationships
Is taught by
teaches
Module
Tutor
teaches
Is taught by
Tutor
Module
teaches
Is taught by
Tutor
Module
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Resolving Many to Many Relationships

• Introduce a link (or weak) entity

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Special cases

• What if, an entity has a relationship with
  itself?
• This happens in family trees and in management
  hierarchies

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Modelling
Person
Is related to
Relationship
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Master Detail

• If you examine a mobile phone bill you could
  split it into two types of information
• A header with your name, address and the phone
  number etc
• The details with the phone number called and the
  associated charges.
• The same is true for many documents purchase
  orders, invoices, goods received notes etc.

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Modelling
Customer
Purchase requested
Header
Product identified
Product
Detail
Product assigned
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Sub-sets

• What if in a given case, only a subset of one
  entity is available for a specific relationship.
• For example, an hotel has many rooms but only a
  subset are allocated for use of the dining room.

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Modelling
Customer
Room
Conference Booking with lunch
Room allocated to dining
Dining suite
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Connection Traps

• Two main types
• Fan Traps
• Chasm Traps

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Fan Traps

• A university has many schools and many
• tutors

• This depiction doesnt show which tutors are
  employed in which school.A better way of showing
  this is

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Chasm Traps

• The diagram in the last slide assumes that
  all tutors are employed by a school. What if
  some tutors are employed by the university to
  work across several schools

• Need to add an additional relationship between
  the University and the Tutor entities.

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Specialisation/Generalisation

• An entity such as Staff is complicated because
  there are usually lots of types of staff within
  an organisation who will potentially relate to
  other entities in different ways
• Eg. Module leader and module tutor
• Each module has one module leader
• A module may have many tutors
• All staff work at the university.

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Superclasses
Staff
leads
Led by
Module
Module leader
allocated
Module Allocation to tutor
allocated
Module tutor
Employed by
employs
University