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Int306 02

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Database design, E-R Model

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Int306 02

  1. 1. INT306 Database Management Systems DATABASE DESIGN & THE E-R MODEL Text book Database System Concepts A. Silberschatz, H. F. Korth, S. Sudarshan Course Instructor Mr. Sumit Mittu Assistant Professor and Placement Coordinator, CSE/IT Lovely Professional University, Punjab (India) sumit.12735@lpu.co.in sumit.mittu@gmail.com http://tinyurl.com/askSumit 01-03-2014 11:42:06
  2. 2. IN THIS CHAPTER • • • • • • • • data modelling overview of database design process entity-relationship model constraints removing redundant attributes in entity sets entity-relationship diagram reduction to relational schema entity-relationship design issues 01-03-2014 11:42:07 Sumit Mittu, Assistant Professor, CSE/IT 2
  3. 3. DATA MODELS • Data Model defined… • Collection of conceptual tools for describing: • • • • Data Data relationships Data semantics, and Consistency constraints • Types • Object Based Logical data models • Record Based Logical data models • Physical data models 01-03-2014 11:42:07 Sumit Mittu, Assistant Professor, CSE/IT 3
  4. 4. DATA MODELS Object based Logical Models Record based Logical Models (Semantic) (Semantic) ER Model 01-03-2014 11:42:07 OO Model Relational Hierarchical Sumit Mittu, Assistant Professor, CSE/IT Network Physical Model Unifying 4
  5. 5. DATA MODELLING • A database can be modelled as: • a collection of entities, and • relationships among entities • An entity: • An object that exists and is distinguishable from other objects • Example: specific person, company, event, plant • Entities have attributes • Example: people have names and addresses • An entity set • A set of entities of same type that share the same attributes • Example: set of all persons, companies, trees, holidays 01-03-2014 11:42:07 Sumit Mittu, Assistant Professor, CSE/IT 5
  6. 6. DATABASE DESIGN PROCESS • Creating a database involves: • Design of database schema • Design of programs that access and update the data • Design of security scheme to control access to data Design of database schema 01-03-2014 11:42:07 Design of programs that access and update the data Sumit Mittu, Assistant Professor, CSE/IT Design of security scheme to control access to data 6
  7. 7. DATABASE DESIGN PROCESS • Design Phases Collect User Requirements • Fully characterize the needs of prospective database users and domain experts Choose Data Model • Choose a data model and translate above requirements into a conceptual schema Specify Functional Requirements • Users describe the kinds of operations (or transactions) to be performed on data Review Conceptual Schema • Designer reviews conceptual schema to ensure that it meets functional requirements Physical Implementation 01-03-2014 11:42:07 • Logical design phase • Physical design phase Sumit Mittu, Assistant Professor, CSE/IT 7
  8. 8. DATABASE DESIGN PROCESS • Pitfalls to avoid during database design process Incompletenes s Redundancy 01-03-2014 11:42:08 Pitfalls Sumit Mittu, Assistant Professor, CSE/IT 8
  9. 9. SCRATCH YOUR MIND!!! • • 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 9
  10. 10. THE ENTITY–RELATIONSHIP MODEL • The E-R Model • Perceives the real world as consisting of basic objects (called entities) and relationships among them • Represents overall logical design of the database • A semantic model • Useful in mapping the meanings and interactions of real world enterprises onto conceptual schema 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 10
  11. 11. THE ENTITY–RELATIONSHIP MODEL • The E-R Model • Three basic concepts of E-R model • Entity Sets • Composed of entities • Relationship Sets • Composed of relationships • Attributes • Properties of entities (entity sets) and relationships (relationship sets) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 11
  12. 12. THE ENTITY–RELATIONSHIP MODEL • Entity • A thing of an object in real world distinguishable from the others • E.g. each employee in an enterprise, each notebook, each wall • Entity may be: concrete or abstract • Entity Set • Collection of entities with same set of properties/attributes • E.g. Set of all employees in an enterprise, set all notebooks in a bag, set of all walls in a building • Individual entities that constitute a set are known as extension of the entity set. • Two or more entity sets may be disjoint (overlapping) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 12
  13. 13. THE ENTITY–RELATIONSHIP MODEL (Entities) (Entity set) (Entity set) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 13
  14. 14. THE ENTITY–RELATIONSHIP MODEL • Attributes • Each entity posses some properties whose values may help identify an entity uniquely over a set of similar entities • E.g. name/address of an employee, no. of pages in notebook, height of a wall • Each entity has a value for each of its attributes • Domain or value-set of an attribute is the set of permissible values for a given attribute for any entity. • E.g. Age can be a positive integer above 18 for a voter entity. 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 14
  15. 15. Types of Attributes THE ENTITY–RELATIONSHIP MODEL Simple Username, Password Composite Name composed of first name and last name Single-valued Registration No., Password Multi-valued Contact No., Subjects Derived Age derived from DoB Null-valued Criminal Record (that may not exist for some records) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 15
  16. 16. THE ENTITY–RELATIONSHIP MODEL 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 16
  17. 17. THE ENTITY–RELATIONSHIP MODEL • Relationship • An association among several entities • Relationship Set • Set of relationships of same type • Mathematically, it is a set R of ordered pairs set on n≥2 (possibly non-distinct) entity sets E1, E2, … En • R = { (e1,e2,…en} | e1 є E1, e2 є E2, … en є En} • Each ordered pair (e1,e2, … en) is a relationship 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 17
  18. 18. THE ENTITY–RELATIONSHIP MODEL Each line above implies a relationship between entity from instructor entity set and that from student entity set 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 18
  19. 19. THE ENTITY–RELATIONSHIP MODEL • Relationship Set • Participation • The association between entity sets in a relationship • Types: Total or Partial • Relationship instance • An association between named entities • Entity’s Role • Function that entity plays in a relationship • Types: Implicit or Explicit • Recursive Relationship Set • Entities of an entity set form relationship with other entities of same entity set 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 19
  20. 20. THE ENTITY–RELATIONSHIP MODEL • Relationship • Types (or degree of relationship): • Binary (degree 2) • Instructor, Student • Ternary (degree 3) • Instructor, Student, Course • N-ary (degree N) • Involving entities from N entity sets • Descriptive attributes • Attributes assigned to relationships • Session Year attribute of Instructor-Student relationship 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 20
  21. 21. THE ENTITY–RELATIONSHIP MODEL 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 21
  22. 22. SCRATCH YOUR MIND!!! • • • 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 22
  23. 23. CONSTRAINTS • Mapping Cardinalities • or, Cardinality ratios express the number of entities to which another entity may be associated via a relationship set. • Types • • • • One to one One to many Many to one Many to many 01-03-2014 11:42:08 1:1 1:N N:1 M:N Husbands :: Wives Fathers :: Children Children :: Mothers Instructors :: Students Sumit Mittu, Assistant Professor, CSE/IT 23
  24. 24. CONSTRAINTS 1:N 1:1 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 24
  25. 25. CONSTRAINTS M:N N:1 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 25
  26. 26. CONSTRAINTS • Participation Constraints • Total • When all entities of an entity participate in relationship set • E.g. Entity Set A (in diagram) • Partial • When only some of the entities participate in the relationship set • E.g. Entity Set B (in diagram) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 26
  27. 27. KEYS • Keys • Help identify a set of attributes that suffice to distinguish entities from each other • Entity Set Keys • • • • Super keys Candidate Keys Primary Key Alternate Keys • Relationship Set Keys • PK(E1) u PK(E2) u … u PK(En) u {a1,a2,…am} 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 27
  28. 28. REMOVING REDUNDANT ATTRIBUTES • Database design using E-R Model • • • • Identify the entity sets Choose appropriate attributes Form the relationship sets Eliminate redundant attributes that may • Exist in multiple entity sets • E.g. instructor_id, instructor_dept_id and instructor_dept_name repeated in instructor and student entity. The instructor_dept_id and instructor_dept_name may be eliminated from student entity set • Exist even when not actually required 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 28
  29. 29. SCRATCH YOUR MIND!!! • • • 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 29
  30. 30. THE E-R DIAGRAMS • E-R Diagram • Graphical representation of the overall logical structure of the database • Basic Structure • • • • Rectangles, Ellipses, Diamonds, Lines, Dashed lines, Arrows Double Lines, Double Diamonds, Double Ellipses Link Labels (for cardinality and roles) 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 30
  31. 31. THE E-R DIAGRAMS • E-R Diagram • Weak entity sets • • • • Identifying or owner entity set Existence dependence Identifying relationship Discriminator attribute 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 31
  32. 32. 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 32
  33. 33. 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 33
  34. 34. 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 34
  35. 35. 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 35
  36. 36. THE E-R DIAGRAMS 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 36
  37. 37. THE E-R DIAGRAMS • E-R Diagram Reduction to Relational Schema • • • • • • Representation of Strong Entity Sets with simple attributes Representation of Strong Entity Sets with complex attributes Representation of Weak Entity Sets Representation of Relationship Sets Redundancy of Schemas Combination of Schemas 01-03-2014 11:42:08 Sumit Mittu, Assistant Professor, CSE/IT 37
  38. 38. THE E-R DIAGRAMS • E-R Diagram Design Issues • • • • Use of entity sets v/s Attributes Use of Entity Sets v/s Relationship Sets Binary v/s n-ary relationships Placement of Relationship Attributes 01-03-2014 11:42:06 Sumit Mittu, Assistant Professor, CSE/IT 38

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