Overview of E-R model. The Entity-Relationship (E-R) model. used for database (conceptual schema) design

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1 The Entity-Relationship (E-R) model Overview of E-R model used for database (conceptual schema) design Lecture topics: basic E-R modelling extensions to E-R modelling designing an E-R schema deriving relational schema from E-R diagrams References: text 3 rd edition: Chapter 3; Chapter 4, sections 1-2, 7-8; Chapter 9, sections 1-2; text 4 th edition: Chapter 3; Chapter 4, sections 1-2, 7-8; Chapter 7; world/enterprise described in terms of: entities attributes relationships visualization: ER-diagram mature methodology (initially described by Chen, 1976) CS 338 E-R Modelling 3-1 CS 338 E-R Modelling 3-2

2 Basic E-R modelling Entity: a distinguishable object Entity set: set of entities of same type students currently enrolled flights offered by Air Canada burglaries in Ontario during 2003 Graphical representation of entity sets:...continued Attributes describe properties of entities E.g. for entities: um, ame, Major, etc. Domain: set of permitted values for an attribute Graphical representation of attributes: Major Burglary um ame Flight CS 338 E-R Modelling 3-3 CS 338 E-R Modelling 3-4

3 continued Relationship: representation of the fact that certain entities are related to each other Relationship set: set of relationships of a given type Graphical representation of relationship sets um students registered in courses passengers booked on flights parents and their children bank branches, customers, and their accounts In order for a relationship to exist, the participating entities must exist RegisteredIn Course ame Courseum CS 338 E-R Modelling 3-5 CS 338 E-R Modelling 3-6

4 ...continued Branchame Branch Accountum Account Recursive relationships and role names Role: the function of an entity set in a relationship set Role name: an explicit indication of a role Balance HomeTeam Address CAB Team Match Location Visitor StreetAddr Customer SI Teamame Locame Customerame CustomerCity Role labels are needed whenever an entity set has multiple functions in a relationship set. CS 338 E-R Modelling 3-7 CS 338 E-R Modelling 3-8

5 Relationships and attributes Relationships may also have attributes: Score Address Constraints in E-R models binary relationship cardinalities general cardinality constraints Team HomeTeam Visitor Match Location primary keys existence dependencies Teamame Locame CS 338 E-R Modelling 3-9 CS 338 E-R Modelling 3-10

6 Binary relationship cardinality relationships between two entity sets, A and B many-to-one (:1): each entity in A can be related to at most one entity in B, but an entity in B may be related to many entities in A E-R diagram notation:...continued one-to-one (1:1): each entity in A can be related to at most one entity in B, and vice versa 1 1 Employee Manages Department A R 1 Employee WorksIn Department 1 B many-to-many (:): an entity can be related to many entities in the other set, and vice versa Employee WorksOn Project similarly: one-to-many (1:) CS 338 E-R Modelling 3-11 CS 338 E-R Modelling 3-12

7 General cardinality constraints Determine lower and upper bounds on the number of relationships of a given relationship set in which a component entity may participate E-R diagram notation: E (lower,upper) R... Primary keys as in relational model, each entity must be distinguishable from any other entity in its set by its attributes Primary key: selection of attributes chosen by designer as a primary identifier of entities in entity set Dnum Department Managerame Dname Budget Takes Course (3,5) (6,100) Firstame Employee Salary Initial Lastame CS 338 E-R Modelling 3-13 CS 338 E-R Modelling 3-14

8 Existence dependencies Sometimes the existence of an entity depends on the existence of another entity If x is existence dependent on y, then y is a dominant entity x is a subordinate entity Transactions are existence dependent on accounts. Balance Account Of 1 Accum Primary keys of subordinate entity sets Weak entity set: an entity set with subordinate entities Strong entity set: an entity set with no subordinate entities primary identifier of weak entity sets combines with primary identifier of associated strong entity set All transactions for a given account have a unique transaction number. Balance Account Of 1 Accum Transaction Transum Transaction Transum Date Amount Date Amount CS 338 E-R Modelling 3-15 CS 338 E-R Modelling 3-16

9 ...continued A weak entity must have an :1 relationship to a distinct entity set Discriminator of a weak entity set: set of attributes that distinguish subordinate entities of the set, for a particular dominant entity Primary key for a weak entity set: discriminator + primary key of entity set for dominating entities ER diagram notation: (distinguishing an identifying relationship) Example E-R diagram Courseum Course Courseame (1,) 1 SectionOf (1,1) Sectionum (1,1) Section (6,50) TaughtBy EnrolledIn Mark (0,) 1 (3,5) Professor 1 Identifying Other 1 Relationship E Relationship Profum Profame um ame CS 338 E-R Modelling 3-17 CS 338 E-R Modelling 3-18

10 Extensions to E-R modeling Structured attributes Aggregation Specialization Generalization Structured attributes Composite attributes: attributes composed of two or more other attributes Multi-valued attributes: attributes that are set-valued Street City Employee Address Province Hobbies PostalCode CS 338 E-R Modelling 3-19 CS 338 E-R Modelling 3-20

11 Aggregation...continued same, without aggregation: Relationships can be viewed as higher-level entities Courseum Accounts are assigned to a given student enrollment. EnrolledIn Course Courseum um EnrolledIn Course CourseAccount ExpirationDate um (0,1) Account UserId CourseAccount ExpirationDate (0,1) Account UserId CS 338 E-R Modelling 3-21 CS 338 E-R Modelling 3-22

12 Specialization A more specialized kind of entity set may be derived from a given entity set Graduate students are students that have a supervisor and a number of degrees. Generalization Two or more existing entity sets can be abstracted as a more general kind of entity set A vehicle abstracts the notion of a car and a truck. PassengerCount umber Licenceum Car MaxSpeed ame Price MakeAndModel Graduate (1,1) SupervisedBy 1 (0,) Professor AxleCount Licenceum Truck Tonnage Degrees Professorame Price MakeAndModel CS 338 E-R Modelling 3-23 CS 338 E-R Modelling 3-24

13 ...continued Designing an E-R schema Licenceum MakeAndModel Vehicle Price usually many ways to design an E-R schema Tonnage Truck Car MaxSpeed points to consider: use attribute or entity set? use entity set or relationship set? degrees of relationships? extended features? AxleCount PassengerCount alternate diagram notations: Vehicle Vehicle isa Truck Car Truck Car CS 338 E-R Modelling 3-25 CS 338 E-R Modelling 3-26

14 Choosing between attributes and entity sets no simple answer! model employees phones by a Phoneumber attribute, or by Phone entity set related to Employee entity set? is it (could it be) a separate object? do we maintain information about it? can several of its kind belong to another single entity? does it make sense to delete such an object? can it be missing from some of the entity set s entities? can it be shared by different entities? yes to any of the above implies introducing a new entity set Choosing between entity sets and relationship sets again no simple answer! Instead of representing accounts as entities, we could represent them as relationships Branchame Branch StreetAddr Customerame Account Customer Balance Accountum SI CustomerCity CS 338 E-R Modelling 3-27 CS 338 E-R Modelling 3-28

15 Use of non-binary relationships Can always represent a relationship on n entity sets with n-1 binary relationships...continued Another representation: Branchame Accountum Branchame Accountum Branch Account Branch Account Balance BranchAcc Balance CABBranch CAB CABAccount CustomerAcc CABCustomer StreetAddr Customer SI StreetAddr Customer SI Customerame CustomerCity Customerame CustomerCity CS 338 E-R Modelling 3-29 CS 338 E-R Modelling 3-30

16 Use of extended E-R features Can improve modularity and abstraction if used with care Excessive use can complicate design A simple methodology recognize entity sets recognize relationship sets and participating entity sets recognize attributes of entity sets and attributes of relationship sets define binary relationship types and existence dependencies define general cardinality constraints, keys and discriminators draw diagram For each step, maintain a log of assumptions motivating the choices, and of restrictions imposed by the choices. CS 338 E-R Modelling 3-31 CS 338 E-R Modelling 3-32

17 Translating E-R diagrams to relational schema General approach is straightforward: each entity set becomes a table each attribute (of an entity) becomes a table column each relationship set becomes either table columns or table by itself Representing strong entity sets Entity set E with attributes a 1,..., a n table E with attributes a 1,..., a n Entity of type E row in table E Primary key of entity set primary key of table Major um ame um ame Major CS 338 E-R Modelling 3-33 CS 338 E-R Modelling 3-34

18 Structured attributes Composite attributes, multi-valued attributes um ame Major Minor Representing Weak Entity Sets Weak entity set E table E Columns of table E include: attributes of the weak entity set attributes of the identifying relationship set (if any) primary key attributes of entity set for dominating entities Surname Givename Primary key of weak entity set primary key of table um Surname Givename Major Minors um Minor CS 338 E-R Modelling 3-35 CS 338 E-R Modelling 3-36

19 ...continued Balance Account Accum 1 Representing relationship sets If the relationship set is an identifying relationship set for a weak entity set then no action needed Date Log Transaction Amount Transum If some component entity set E has general cardinality constraint (1,1), add columns to table E: attributes of the relationship set primary key attributes of remaining component entity sets Account Accum Balance Transaction Transum Accum Date Amount CS 338 E-R Modelling 3-37 CS 338 E-R Modelling 3-38

20 ...continued...continued Otherwise: relationship set R table R Date TD1Category TD1 Deduction (1,1) Employee Empum Columns of table R include: attributes of the relationship set primary key attributes of each component entity set CPP EI IncomeTax EHT Primary key of table R: if some component entity set E has the general cardinality constraint (0,1), choose the primary key attributes for E otherwise, choose the primary key attributes of each component entity Employee Empum Date TD1Category TD1 TD1Category CPP EI EHT IncomeTax CS 338 E-R Modelling 3-39 CS 338 E-R Modelling 3-40

21 ...continued...continued Score Address (1,) Preference Registered Course Cum ame Sum ame HomeTeam Team Match Visitor Teamame Location Locame Sum ame Cardinality (1,): Cardinality (0,1): Course Cum ame Registered Sum Cum Preference Sum Cum Preference Match Location Locame Address Team Teamame HomeTeamame VisitorTeamame Locame Score ote that the role name of a component entity set is prepended to its primary key attributes CS 338 E-R Modelling 3-41 CS 338 E-R Modelling 3-42

22 Representing aggregation...continued Courseum Tabular representation for aggregation of relationship set R = tabular representation for relationship set R EnrolledIn Course To represent relationship set involving aggregation of R, treat the aggregation like an entity set whose primary key = primary key of the table for R um 1 CourseAccount 1 Account ExpirationDate UserId um EnrolledIn um Course Courseum Courseum Account UserId CourseAccount UserId um Courseum ExpirationDate CS 338 E-R Modelling 3-43 CS 338 E-R Modelling 3-44

23 Representing specialization Create specialized table with primary attribute of higher-level entity, plus attributes as usual Representing generalization Create a table for each lower-level entity set only umber ame Columns of new tables should include: Attributes of lower level entity set Attributes of the superset Graduate (1,1) SupervisedBy 1 (0,) Professor The higher-level entity set can be defined as a view on the tables for the lower-level entity sets Degrees Professorame umber ame Graduate umber Professorame Professor Professorame Degree umber Degree CS 338 E-R Modelling 3-45 CS 338 E-R Modelling 3-46

24 ...continued MakeAndModel Licenceum Vehicle Price Tonnage Truck Car MaxSpeed AxleCount PassengerCount Truck Licenceum MakeAndModel Price Tonnage AxleCount Car Licenceum MakeAndModel Price MaxSpeed P gercount CS 338 E-R Modelling 3-47

Data Modeling and the Entity-Relationship Model

Data Modeling and the Entity-Relationship Model David Toman School of Computer Science University of Waterloo Introduction to Databases CS348 David Toman (University of Waterloo) ER Model 1 / 29 Overview