1. Topic 3: ER – Entity Relationship Model (ERM) 6/12/2016 1

2. Example ER An ER diagram represents several assertions about the real world. What are they? When attributes are added, more assertions are made. How can we ensure they are correct? A DB is judged correct if it captures ER diagram correctly. Students Professor teaches Department faculty major offers Courses enrollment advisor

3. Database Design Process Requirement collection and analysis ◦ DB requirements and functional requirements Conceptual DB design using a high-level model ◦ Easier to understand and communicate with others Logical DB design (data model mapping) ◦ Conceptual schema is transformed from a high-level data model into implementation data model Physical DB design ◦ Internal data structures and file organizations for DB are specified

4. Overview of Database Design Conceptual design: (ER Model is used at this stage.) ◦ What are the entities and relationships in the enterprise? ◦ What information about these entities and relationships should we store in the database? ◦ What are the integrity constraints or business rules that hold? ◦ A database `schema’ in the ER Model can be represented pictorially (ER diagrams). ◦ An ER diagram can be mapped into a relational schema.

5. ER 1976 proposed by Peter Chen ER diagram is widely used in database design ◦Represent conceptual level of a database system ◦Describe things and their relationships in high level 6/12/2016 5

6. Basic Concepts Entity: Real-world object distinguishable from other objects Entity set – an abstraction of similar things, e.g. cars, students ◦An entity set contains many entities Attributes: common properties of the entities in a entity sets Relationship – specify the relations among entities from two or more entity sets 6/12/2016 6

7. Basic Concepts… Relationship: Association among two or more entities. e.g., Jack works in Pharmacy department. Relationship Set: Collection of similar relationships. ◦ An n-ary relationship set R relates n entity sets E1... En; each relationship in R involves entities e1 in E1,..., en in En ◦Same entity set could participate in different relationship sets, or in different “roles” in same set. 6/12/2016 7

8. ER Model Basics Key and key attributes: ◦Key: a unique value for an entity ◦Key attributes: a group of one or more attributes that uniquely identify an entity in the entity set Super key, candidate key, and primary key ◦ Super key: a set of attributes that allows to identify and entity uniquely in the entity set ◦ Candidate key: minimal super key ◦ There can be many candidate keys ◦ Primary key: a candidate key chosen by the designer ◦ Denoted by underlining in ER attributes

9. Difference between Candidate Key and Primary key Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key. Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key. 6/12/2016 9

10. An Example 6/12/2016 10

11. Relationship A relationship may be thought as a set as well ◦For binary relationship, it enumerates the pairs of entities that relate to each other ◦For example, entity set M = {Mike, Jack, Tom} entity set F = {Mary, Kate}. The relationship set married between M and F may be {, } 6/12/2016 11

12. Relationship 6/12/2016 12 A relationship set is a mathematical relation among n  2 entities, each taken from entity sets A relationship set is a mathematical relation among n  2 entities, each taken from entity sets {(e 1, e 2, … e n ) | e 1  E 1, e 2  E 2, …, e n  E n } where (e 1, e 2, …, e n ) is a relationship Example: Example: (Hayes, A-102)  depositor

13. Relationship Example 6/12/2016 13

14. Attribute of A Relationship Set 6/12/2016 14

15. Relationship The degree of a relationship = the number of entity sets that participate in the relationship ◦Mostly binary relationships ◦Sometimes more Mapping cardinality of a relationship ◦1 –1 ◦1 – many ◦many – 1 ◦Many-many 6/12/2016 15

16. Key Constraints Consider Works_In: An employee can work in many departments; a dept can have many employees. In contrast, each dept has at most one manager, according to the key constraint on Manages. Many-to-Many 1-to-11-to ManyMany-to-1 dname budgetdid since lot name ssn Manages Employees Departments

17. One-One and One- Many 6/12/2016 17

18. Many-one and many- many 6/12/2016 18

19. 1- many 6/12/2016 19

20. Many - 1 6/12/2016 20

21. Many - many 6/12/2016 21

22. Alternative Cardinality Specification 6/12/2016 22

23. Note on Mapping Cardinality Both many and 1 include 0 ◦Meaning some entity may not participate in the relationship 6/12/2016 23

24. Participation Constraints Does every department have a manager? ◦ If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). ◦Every Departments entity must appear in an instance of the Manages relationship. lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

25. Total Participation 6/12/2016 25 When we require all entities to participate in the relationship (total participation), we use double lines to specify Every loan has to have at least one customer

26. Self Relationship Sometimes entities in a entity set may relate to other entities in the same set. Thus self relationship Here employees manage some other employees The labels “manager” and “worker” are called roles the self relationship 6/12/2016 26

27. More examples on self- relationship People to people ◦Parent – children ◦Manager – employee ◦Husband – wife Word to word ◦Root – synonym 6/12/2016 27

28. Attributes Both entity sets and relationships can have attributes Attributes may be ◦Composite ◦Multi-valued (double ellipse) ◦Derive (dashed ellipse) 6/12/2016 28

29. Another Example 6/12/2016 29

30. Keys A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity. A candidate key of an entity set is a minimal super key Although several candidate keys may exist, one of the candidate keys is selected to be the primary key. 6/12/2016 30

31. Key Examples Suggest super keys for the following entity? What are the candidate keys? Primary key? 6/12/2016 31 author name birthday death description

32. Ternary Relationship 6/12/2016 32

33. Can We Decompose a Ternary Relationship? Some relationships that appear to be non-binary may be better represented using binary relationships ◦E.g. A ternary relationship parents, relating a child to his/her father and mother, is best replaced by two binary relationships, father and mother ◦Using two binary relationships allows partial information (e.g. only mother being know) ◦But there are some relationships that are naturally non- binary ◦E.g. works-on, why? 6/12/2016 33

34. Converting Ternary to binary In general, any non-binary relationship can be represented using binary relationships by creating an artificial entity set. ◦Replace R between entity sets A, B and C by an entity set E, and three relationship sets: 1. R A, relating E and A 2.R B, relating E and B 3. R C, relating E and C ◦Create a special identifying attribute for E ◦Add any attributes of R to E ◦For each relationship (a i, b i, c i ) in R, create 1. a new entity e i in the entity set E 2. add (e i, a i ) to R A 3. add (e i, b i ) to R B 4. add (e i, c i ) to R C 6/12/2016 34

35. Converting Ternary to binary 6/12/2016 35

36. Design an ER Diagram Design a database for an on-line reservation system for microscopes in material science lab There are two types of users: microscope administrators and microscope end users Each microscope is located in a specific lab Each request is assigned to an administrator who can authorize or deny the request Using of some microscope requires the presence of an administrator Time is divided into 1 hour slots. Each reservation can only take one or more time slots 6/12/2016 36

37. Weak Entity Set Some entity sets in real world naturally depend on some other entity set ◦They can be uniquely identified only if combined with another entity set Example: ◦section1, section2, … become unique only if you put them into a context, e.g. csce4350 6/12/2016 37

38. Weak Entity Set Notations 6/12/2016 38 Double rectangles for weak entity set Double diamond for weak entity relationship Dashed underscore for discriminator

39. Specialization A lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked. A lower-level entity set may have additional attributes and participate in additional relationships 6/12/2016 39

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41. Specification Disjoint Completeness constraint (use double lines) ◦total : an entity must belong to one of the lower-level entity sets ◦partial: an entity need not belong to one of the lower-level entity sets 6/12/2016 41

42. Design Considerations Use of entity sets vs. attributes ◦Whether we want to keep additional information Use of entity sets vs. relationship sets ◦Actions among entities are usually represented by relationships Binary versus n-ary relationship sets ◦N-nary relationships are usually more natural for actions among entity sets Weak entity set vs. strong entity set Generalization 6/12/2016 42

43. Notations 6/12/2016 43

44. Notations 6/12/2016 44

45. ER Practice Again Design an ER diagram for an online music store. The database will contain at least the following concepts: songs, artists, bands, albums, and genres. State your design assumptions you make to support design decisions. Be sure your assumptions are reasonable. 6/12/2016 45

46. Best Practice Guide for ER Design Use of entity sets vs. attributes Use of entity sets vs. relationship sets Binary versus n-ary relationship sets Weak entity set vs. strong entity set ◦Choose the natural one Generalization ◦If specialized entities need to keep additional information and participate in additional relationships 6/12/2016 46

47. ER for Banking Enterprise Description handhout 6/12/2016 47

48. Read ER Diagrams Following are some ER diagrams grabbed from the web Read to understand/criticize 6/12/2016 48

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