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Lecture plan

Lecture plan. Outline of DB design process Entity-relationship model Entities and attributes Relationships. DB design process [1]. Requirements collection and analysis Users’ data requirements Users’ functional requirements (using software engineering tools, e.g. data flow diagrams).

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Lecture plan

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  1. Lecture plan • Outline of DB design process • Entity-relationship model • Entities and attributes • Relationships

  2. DB design process [1] • Requirements collection and analysis • Users’ data requirements • Users’ functional requirements (using software engineering tools, e.g. data flow diagrams)

  3. DB design process [2] • Conceptual design • Creation of conceptual schema • Concise description of users’ data requirements • Uses high-level conceptual data model, e.g. Entity-relationship model • Data model operations used to specify user operations from functional analysis • Compatibility check and possible modification

  4. DB design process [3] • Logical design / data model mapping • Uses implementation data model, e.g. relational data model • Conceptual schema transformed from high-level data model to implementation data model

  5. DB design process [4] • Physical design • Internal storage structures, access paths, file organisation specified • Application programs designed and implemented

  6. Entity-relationship model • High-level conceptual modelling technique for DB applications • DB application = DB + associated programs • Application programs need to be designed, implemented and tested • ER model used for conceptual design of DB applications

  7. Entities and attributes • An entity is an object with a physical or conceptual existence • Attributes are the particular properties which describe the entity • An individual attribute has a value

  8. Simple vs composite attributes • Composite attributes • Can be divided into smaller subparts which • Represent more basic attributes with independent meanings • Can form a hierarchy • Useful if attribute must be handled sometimes as a unit and sometimes as components • Simple (atomic) attributes are not divisible

  9. Single- vs multi-valued attributes • Single-valued attributes have a single value for a particular entity • Multi-valued attributes • Have a set of values for a particular entity • May have lower/upper bounds on the number of values allowed

  10. Stored vs derived attributes • Derived attributes • Are related to another attribute • Are derivable from a stored attribute • May be derivable from related entities • Stored attributes • Are not derivable from any other source

  11. Null attribute values • Used to denote value of an attribute for a particular entity which is • Non-applicable • Unknown • Exists but is missing • Not known if it exists or not

  12. Complex attributes • Composite attributes • Components grouped between parentheses () • Components separated by commas • Multi-valued attributes • Displayed between braces {} • Composite and multi-valued attributes can be nested in an arbitrary way

  13. Entity types and sets [1] • An entity type is • A collection (set) of entities with the same attributes but different values for the attributes • Schema / intension for a set of entities which share the same structure

  14. Entity types and sets [2] • An entity set • A collection of all entities for a particular entity type • Extension of the entity type • Entity type and associated entity set have the same name

  15. Entity types and sets [3] EMPLOYEE Name, Age, Salary e1 (John Smith, 55, 80K) e2 (Fred Brown, 40, 30K) e3 (Judy Clark, 25, 20K)

  16. Key attributes [1] • Also called uniqueness constraint • Value of key attribute • Identifies each entity uniquely • Must be distinct for each entity in the collection • Unique for every extension of the entity type

  17. Key attributes [2] • Composite key • Combination of attribute values form the key • Must be minimal (no superfluous attributes) • Entity type may have • More than one key attribute • No key attribute - a weak entity type

  18. Value sets of attributes • Also called “domain” of attributes • The set of values which may be assigned to that attribute for each individual entity • Not displayed in ER diagrams

  19. Relationships • Implicit relationships between attributes can be converted to explicit relationships • An attribute of one entity type refers to another entity type

  20. Relationship types and sets • Relationship type defines set of associations (relationship set) among entities • Relationship type and set have same name • Relationship instance associates exactly one entity from each participating entity type

  21. Relationship degree • Number of participating entity types: • Relationship type of degree two is binary • Relationship type of degree three is ternary • Higher degree relationships are more complex

  22. Relationships as attributes • Two entity types A and B, where • Attribute a2 of entity type A indicates a link to a particular entity of entity type B • Binary relationship may be considered as: • Value set of attribute a2 is set of all B entities • Additional multi-valued attribute b3 for entity type B, whose value for each entity is the set of all A entities matching on attribute a2

  23. Role names • Explains what the relationship means • Indicates the role that a particular entity plays in a relationship instance • Necessary for recursive relationships • same entity type participates more than once in a relationship type in different roles

  24. Relationship type constraints [1] • Structural constraints which limit combination of entities in relationship set • Cardinality ratio • Number of relationship instances an entity can participate in • Possible ratios - 1:1, 1:N, N:1, M:N

  25. Relationship type constraints [2] • Participation constraints • Indicates if existence of an entity depends on its being related to another entity by relationship type • Total (existence dependency) - every entity in the total set must be related • Partial - some entities in the total set must be related

  26. Relationship type attributes • Similar idea to entity type attributes • Attributes of 1:1 relationship types can be migrated to one of participating entities • Attributes of 1:N (or N:1) relationship types can be migrated to entity type at N side • Attributes of M:N relationship types cannot be migrated

  27. Weak entity types [1] • Does not have a key attribute • Identified by a combination of • Relationship to specific entities from another entity type (identifying/owner entity type) • Identifying relationship of weak entity type • Weak entity has total participation constraint • Some (or all) of its attribute values

  28. Weak entity types [2] • Normally has partial key (discriminator) • Can sometimes be represented as complex (composite, multi-valued) attribute • Not if it participates independently in other relationship types

  29. ER conceptual design • Schema design is iterative and may involve refinement: • Attribute may be refined into relationship if attribute is a reference to another entity type • Attribute that exists in several entity types may be refined into an independent entity type • Inverse refinement to above • Specialisation/generalisation

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