Ch.6 DB design. Why design? Pitfalls at the two extremes: Redundancy Incompleteness. Two strategies: Bottom-up Top-down. Ch.6 is devoted to bottom-up

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1 Week 7, Lect.3 SKIP CH.5 Other relational languages procedural o Tuple relational calculus o Domain relational calculus non-procedural o QBE (Query By Example) o Datalog

2 Ch.6 DB design Why design? In order to deal w/complexity use layered/hierarchical approach. Pitfalls at the two extremes: Redundancy Incompleteness Two strategies: Bottom-up Top-down Ch.6 is devoted to bottom-up

3 6.2 The E-R model So, what is at the bottom of (relational) DB design? Entities = things Entity sets not necessarily disjoint or covering: E.g. a person can be both a customer and an employee of the bank E.g. a person can be neither employee nor customer (resist the temptation of representing EVERYTHING, we have to draw the line somewhere, e.g. ) From a DB perspective: Entities = sets of attributes Attribute = property of an entity, takes on values

5 In a relational DB: Relationships Relationship sets

6 Subset of the full cartesian product E1 x E2 x x En

7 READ and take notes: Section Attributes

8 6.3 Constraints Types of relationships sets: By the number of entities at each end of the relation, a.k.a. mapping cardinality:

9 Attention: The one should be understood as at-most-one! Participation constaints further cast light in this matter: o Total participation o Partial participotion Give examples of each of the 4 types

10 Keys needed to uniquely identify entities Superkey Candidate key Primary key Further problem: How to distinguish relationships? A: Just take the union of the primary keys of all the entity sets involved

11 6.4 E-R diagrams multivalued and derived attributes

12 A relationship set can involve the same entity set under multiple roles: Directed arrows one-to-many (tip points towards 1 )

13 Conversely, a relationship set can involve more than 2 entity-sets

14 One-to-many Many-to-one

15 Many-to-many One-to-one

16 Total participation of loan into borrower More detailed cardinality limits

17 Week 8, Lect E-R design issues Entity set or attribute?

18 Entity set or relationship set?

19 Entity sets vs. relationship sets example Write the relations needed in loan to create loans for Mr. Asimov and Mr. Banks at the Dublin branch. Use arbitrary loan numbers and amounts (be generous ) Now create an account in Stephenville that is jointly owned by all 3 customers o Write the new relations o Comment on redundancy/replication o What do we need to do in order to update the amount?

20 Binary or non-binary relationship sets "Turn of the crank" method to turn any n-ary relationship into n binary ones (a, b, c) (e, a) + (e, b) + (e, c) Disadvantages: extra attribute e requires extra storage the meaning of the n- ary relationship becomes hard to see some constraints on n- ary are impossible to represent w/only binary

21 Exercise: add constraints to the relationships RA, RB and RC to ensure that a newly created entity corresponds to exactly one entity in each of entity sets A, B and C Exercise: add constraints to the relationships RA, RB and RC to ensure that a newly created entity in A relates to exactly one entity in B or C, but not in both.

22 Placement of relationship attributes Where to put access_date? Draw the possible E-R diagrams. What if the relationship is One-to-one? Many-to-one? Many-to-many?

23 This concludes the material required for the midterm. Last page in text: 225 Wednesday REVIEW

24 Week 8, Lect.2 Review for midterm

25 Week 8, Lect.3 Midterm

A database can be modeled as: + a collection of entities, + a set of relationships among entities.

A database can be modeled as: + a collection of entities, + a set of relationships among entities. The Relational Model Lecture 2 The Entity-Relationship Model and its Translation to the Relational Model Entity-Relationship (ER) Model + Entity Sets + Relationship Sets + Database Design Issues + Mapping