2. E-R Model. Entity Sets Relationship Sets Attributes

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1 3. E R Model

2 2. E-R Model Entity-Relationship Model Graphical Representation of Database Equivalent to Flow-Chart in Programming It makes easy to understand Database Prior Step to implement Actual Database Designer s First Choice It consist Entity Sets Relationship Sets Attributes

3 Entity Entity is a thing or object which is unique in the world e.g. each student in the college is an entity Entity has a set of properties called attributes or values for some set of properties may uniquely identify an entity. e.g. each student having rollno, name, sem, branch, ph, address, age Entity Set is a set of entities of the same type that share the same attributes or properties e.g. the set of all the students having the same attributes is an entity set

4 Attributes Nature of the things or objects An entity is basically represented by a set of Attributes An each entity set has the entities of having the same Attributes Each entity has the value for each of its Attribute For each attribute, there is a set of permitted values called Domain or Value Set Each entity in an entity set described by a set of (attribute, data value) pairs

Simple and Composite Attribute Types Simple(Atomic) attributes

custid Composite attributes are divided into subparts e.g.

Multi-valued Single-valued attribute has only one value from

values from specific domain.( 2 phone no.

5 Simple and Composite Attribute Types Simple(Atomic) attributes are not divided into subparts e.g. custid Composite attributes are divided into subparts e.g. cust-name (firstname, middlename, lastname) Single-valued and Multi-valued Single-valued attribute has only one value from specific domain Multi-valued attribute has zero, one or more values from specific domain.( 2 phone no. per person) Stored & Derived Value of Derived attribute can be derived from values of other attributes (stored).e.g. DOB & age

6 E-R Diagram With Composite, Multivalued, and Derived Attributes

7 E-R Diagram Symbols name street number amount id city customer m:n 1:n n:1 1:1 borrower loan entity set attribute relationship set weak primary key multivalued derived Total participation

8 Relationship Set Logical connectivity or Association among several entities Relationship Set (Relation) is a set of relationships of the same type If E 1, E 2,,E n are entity sets, Then a relationship set R is a subset of {(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

9 Role of an Entity in Relationship Function of an entity in the relationship is called Role Same entity can participate in a relationship with a different roles. Such type of relationship set is called recursive relationship set, in which explicit role names are necessary to specify employee.employee work-for manager.employee Id 1 Anand Name Id Name 2 Milind 3 Suraj 5 Rajesh 4 Ketan 5 Rajesh 7 Jay 6 Manish 7 Jay

10 Relationship s Attribute Descriptive attribute e.g. depositor relationship set with entity sets customer and account. Set the access-date with depositor relationship to specify most recent data on which customer accessed an account customer depositor account name access-date balance

11 Recursive Relationship Same entity participate more than once in relationship type with different roles e.g. supervision relationship set relates an Employee Set to a supervisor, where both employee & supervisor entities are members of same employee entity type. So employee participate in 2 role, one as boss & other supervisee. supervisor 1 Employee Supervision supervisee N

Mapping Cardinalities Mapping cardinalities, or cardinality ratios, express the number of entities to which another entity can be associated via a relationship set Best Beneficial for describing

12 Mapping Cardinalities Mapping cardinalities, or cardinality ratios, express the number of entities to which another entity can be associated via a relationship set Best Beneficial for describing Binary Relationship Relationship set = R entity sets = A & B One to one: An entity in A is associated with at most one entity in B and visa versa Employee---manages--dept One to many or Many to one: An entity in A is associated with any no.(zero or more) of entities in B but an entity in B is associated with at most one entity in A Employee ---works for--dept Many to Many: An entity in A is associated with any no. (zero or more) of entities in B and visa versa employee works on--projects

13 A one:one B A many:many B a 1 b 1 a 1 b 1 a 2 a 3 b 2 b a 2 a 3 b 2 b 3 a 4 b 4 a 4 b 4 Mapping Cardinalities A B A B a 1 a 2 a 3 b 1 b 2 b 3 b a 1 a 2 a 3 a 4 b 1 b 2 b 3 one:many many:one

Participation Constraints Total Participation Entity set E in a relationship set R is said to be a total If every entity in E participates in at least one relationship in R

14 Participation Constraints Total Participation Entity set E in a relationship set R is said to be a total If every entity in E participates in at least one relationship in R Partial Participation If some entities of E participate in Relationships in R, then the participation of E is said to be partial shown by single line) customer borrower loan

15 E-R Diagram with a Ternary Relationship

18 University ER Diagram Degree Birth date Name StudentID SSN Major In DName DCode OfficeNumber Sex Student Department OfficePhone Class Minor In College Address City State Zip Offer CName Letter Grade Grade_Report Instructor Year Course CourseDesc GPA CNumber Numeric Grade Section Belong_To Credits SectionNumber Semester

19 Super Key Candidate Key Keys Allows us to uniquely identify an Entity in an Entity Set Made up of set of one or more attributes of an Entity Set Set of one or more attributes, taken collectively, allow us to use as key of an Entity Set e.g. {id} is a superkey for customer entity set e.g. {name, street} collectively is also a superkey for customer entity set K is a superkey then superset of K is also a superkey Superkey, for which, no proper subset is a superkey e.g. {id} is a candidate key for customer e.g. {name,street} also acts as a candidate key

20 Primary Key Keys (Cont ) A candidate key which is chosen as a key of an Entity Set by Database Designer as the principal means of identifying entities within an Entity Set is called a Primary Key e.g. {id} acts as a primary key for customer A Key (primary, candidate, super) is a property of the Entity Set, rather than of an individual entities Primary key should be chosen as set of attributes which are never or very rarely changed

21 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 Customer_id is candidate key of customer account_number is candidate key of account Although several candidate keys may exist, one of the candidate keys is selected to be the primary key.

22 Keys for Relationship Sets The combination of primary keys of the participating entity sets forms a super key of a relationship set. (customer_id, account_number) is the super key of depositor NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set. Example: if we wish to track all access_dates to each account by each customer, we cannot assume a relationship for each access. We can use a multivalued attribute though Must consider the mapping cardinality of the relationship set when deciding what are the candidate keys Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key

23 Weak Entity Sets An entity set that does not have a primary key is referred to as a weak entity set. The existence of a weak entity set depends on the existence of a identifying entity set it must relate to the identifying entity set via a total, one-tomany relationship set from the identifying to the weak entity set Identifying relationship depicted using a double diamond The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set. The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set s discriminator.

24 Weak Entity Sets (Cont.) We depict a weak entity set by double rectangles. We underline the discriminator of a weak entity set with a dashed line. payment_number discriminator of the payment entity set Primary key for payment (loan_number, payment_number)

25 Weak Entity Sets (Cont.) Note: the primary key of the strong entity set is not explicitly stored with the weak entity set, since it is implicit in the identifying relationship. If loan_number were explicitly stored, payment could be made a strong entity, but then the relationship between payment and loan would be duplicated by an implicit relationship defined by the attribute loan_number common to payment and loan

26 More Weak Entity Set Examples In a university, a course is a strong entity and a course_offering can be modeled as a weak entity The discriminator of course_offering would be semester (including year) and section_number (if there is more than one section) If we model course_offering as a strong entity we would model course_number as an attribute. Then the relationship with course would be implicit in the course_number attribute

29 Hotel mgmt

31 Extended E-R Features: Specialization Top-down design process; we designate subgroupings within an entity set that are distinctive from other entities in the set. These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set. Depicted by a triangle component labeled ISA (E.g. customer is a person). Attribute inheritance a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.

32 Specialization Example

33 Extended ER Features: Generalization A bottom-up design process combine a number of entity sets that share the same features into a higher-level entity set. Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way. The terms specialization and generalization are used interchangeably.

34 Specialization and Generalization (Cont.) Can have multiple specializations of an entity set based on different features. E.g. permanent_employee vs. temporary_employee, in addition to officer vs. secretary vs. teller Each particular employee would be a member of one of permanent_employee or temporary_employee, and also a member of one of officer, secretary, or teller The ISA relationship also referred to as superclass - subclass relationship

35 Aggregation Consider the ternary relationship works_on, which we saw earlier Suppose we want to record managers for tasks performed by an employee at a branch

36 Aggregation (Cont.) Relationship sets works_on and manages represent overlapping information Every manages relationship corresponds to a works_on relationship However, some works_on relationships may not correspond to any manages relationships So we can t discard the works_on relationship Eliminate this redundancy via aggregation Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity Without introducing redundancy, the following diagram represents: An employee works on a particular job at a particular branch An employee, branch, job combination may have an associated manager

37 E-R Diagram With Aggregation

38 Mapping E-R to Relational Model 1.Mapping of regular entity 2.Mapping of weak entity 3.Mapping of binary (1:1) relationship types 4.Mapping of binary(1:n) or (N:1) relationship types 5.Mapping of binary (M:N) types 6. Mapping of multivalued Attributes 7.Mapping if ISA relationship 8.Mapping of n-ary relationship types

40 1. Mapping of regular entity 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance)

42 2. Mapping of weak entity 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4.Loan ( loan_num, amount) 5.Account (act_num, balance) 6. Payment ( loan_num, payment_num, pay_date, pay_amt)

44 3. Mapping of 1:1 relationship 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) E1 R E2 IN 1:1 constraints of entity e1 & e2. choose entity having total participation.add primary key of e1 to e2 & also add descriptive attributes to e2.

45 4. Mapping of 1:N relationship 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Acc_branch (acct_num, bname) 8. Loan_branch ( loan_num, bname) 9. Cust_bank ( cid, eid, type) 10.Works for ( work_eid, manager_eid) E1 1 N R E2 IN 1:N constraints of entity e1 & e2. choose entity having 1 participation add primary key of 1 to N entity. Or create new entity named by that relationship

46 5. Mapping of M:N relationship 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Acc_branch (acct_num, bname) 8. Loan_branch ( loan_num, bname) 9. Cust_bank ( cid, eid, type) 10. Works for ( work_eid, manager_eid) 11.Borrower( cid,loan_num) 12.Depositer (cid, acct_num) E1 M R N E2 A IN M:N constraints of entity e1 & e2. Create new entity e3 add primary key of e1 & e2 & descripitive attribute of that relationship if it have.

47 6. Mapping of Multivalued 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Acc_branch (acct_num, bname) 8. Loan_branch ( loan_num, bname) 9. Cust_bank ( cid, eid, type) 10. Works for ( work_eid, manager_eid) 11. Borrower( cid,loan_num) 12. Depositer (cid, acct_num) 13.Dependent_name ( eid,dname) E2 A Create a new entity which include its primary key & that attribute

49 7. Mapping of ISA 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Acc_branch (acct_num, bname) 8. Loan_branch ( loan_num, bname) 9. Cust_bank ( cid, eid, type) 10. Works for ( work_eid, manager_eid) 11. Borrower( cid,loan_num) 12. Depositer (cid, acct_num) 13.Dependent_name(eid, dname) 14.Saving_acct ( act_num, intrest_rate)) 15.Check_act(act_num, amt)

50 8. Mapping of N-ary relationship 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) E1 R E2 A E1 Create a new entity including all primary key of entity & attribute if any.

51 Reduction 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount) 5. Account (act_num, balance) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Acc_branch (acct_num, bname) 8. Loan_branch ( loan_num, bname) 9. Cust_bank ( cid, eid, type) 10. Works for ( work_eid, manager_eid) 11. Borrower( cid,loan_num) 12. Depositer (cid, acct_num) 13.Dependent_name(eid, dname) 14.Saving_acct ( act_num, intrest_rate)) 15.Check_act(act_num, amt)

52 reduction 1. Customer (cid, cname, cstreet, ccity) 2. Employee ( eid, ename, telephone,startdate, mgr_eid) 3. Branch ( bname, bcity, assets) 4. Loan ( loan_num, amount,bname) 5. Account (act_num, balance,bname) 6. Payment(loan_num,payment_num, pay_date, pay_amt) 7. Cust_bank ( cid, eid, type) 8. Borrower( cid,loan_num) 9. Depositer (cid, acct_num) 10.Dependent_name(eid, dname) 11.Saving_acct ( act_num, intrest_rate)) 12.Check_act(act_num, amt)

Chapter 6: Entity-Relationship Model

Chapter 6: Entity-Relationship Model Database System Concepts, 5th Ed. See www.db-book.com for conditions on re-use Chapter 6: Entity-Relationship Model Design Process Modeling Constraints E-R Diagram