In this lecture we look at...
[Section notes PDF 117Kb]

• Design/schema side (Entity types)
• Object-orientated concepts
• Java, C++ or UML
• Sub/superclasses and inheritance
• EER diagrams
• EER to Relational mapping

• Inheritance concept
• Attributes (and methods)
• Subtypes and supertypes
• Specialisation and Generalisation
• ER diagrams
• show entities/entity sets
• EER diagrams
• show type inheritance
• additional 8^th step to ER→Relational mapping

• Basic guide to Java
• Object, classes as blueprints
• Object, collection of methods and attributes
• Miniworld model of real world things
• Object, entity in database terms

• Similar objects
• Car Park example
• Student example
• Shared properties/attributes
• Generalisation
• Reverse, specialisation

• Using English as model
• ‘Is a’ (inheritance)
• ‘Has a’ (containment)
• Nouns as objects
• Verbs as methods
• Adjectives as variables (sort of)

• Superclasses (Student)
• Subclasses (Engineer, Geographer, Medic)
• Inheritance
• Subclass inherits superclass attributes
• Union of specific/local and general attributes
• Inheritance chains
• Person → Student → Engineer → Computer Scientist

• Partial participation
• Disjoint subclasses
• A fruit may be either a pear or an apple or a banana, or none of them. A fruit may not be a pear and a banana, an apple and a banana, an apple and a pear …

• Total, disjoint
• Equivalent to Java Abstract classes
• A Wine has to be either Red, White or Rosé cannot be both more

• Specialisation lattices
• and Hierarchies
• Multiple inheritance
• Union of two superclasses (u in circle)
• In addition to basic ER notation

• Subset symbol to illustrate
• sub/superclass relationship
• direction of relationship
• Circle to link super to subclasses
• Disjoint
• Overlapping
• Union

• Disjointness (d in circle) – single honours
• Overlapping (o in circle) – joint honours/sports
• Membership condition on same attribute
• attribute-defined specialisation
• defining attribute
• implies disjointness
• versus user-defined
• each entity type specifically defined by user

• Total specialisation
• Every entity in the superclass must be a member of atleast 1 subclass
• Double line (as ER)
• Partial specialisation
• Some entites may belong to atleast 1 subclass, or none at all
• Single line
• Yields 4 possibilities
• (Total-Dis, Total-Over, Partial-Dis, Partial-Over)

• Total, overlapping
• A Chip may has to be at least one of FPA Unit, Reg Block, L1 Cache, and may be more than one type

• Type hierarchies
• Specialisation lattices
• Well, sir, the Supreme Court of the United States has determined that the tomato is for legal and commercial purposes both a fruit and a vegetable. So we can legally refer to tomato juice as 'vegetable' juice.
• Candice, General Foods

• Initially following 7 ER stages
• Stage 8
• 4 different options
• Optimal solution based on problem
• Let C be superclass, S_1..m subclasses

• Create relation for C, and relations for S_1..m each with a foreign key to C (primary key)
• Create relations for S_1..m each including all attributes of C and its primary key

• Create a single relation including all attributes of C ∪ S_1..m and a type/discriminating attribute
• only for disjoint subclasses
• Create a single relation as above, but include a boolean type flag for each subclass
• works for overlapping, and also disjoint