# ENTITY-RELATIONSHIP MODEL. CS 564- Fall 2016

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1 ENTITY-RELATIONSHIP MODEL CS 564- Fall 2016

2 HOW TO BUILD A DB APPLICATION Pick an application Figure out what to model (ER model) Output: ER diagram Transform the ER diagram to a relational schema Refine the relational schema (normalization) Now ready to implement the schema and load the data! CS 564 [Fall 2016] - Paris Koutris 2

3 RUNNING EXAMPLE We want to store information about: companies and employees Each company has a name, an address, Each company has a list of employees products manufactured by these companies Each product has a name, a description, CS 564 [Fall 2016] - Paris Koutris 3

4 THE ER MODEL Gives us a language to specify what information the DB must hold what are the relationships among components of that information Proposed by Peter Chen in 1976 What we will cover: 1. basic stuff: entities, attributes, relationships 2. constraints 3. weak entity sets 4. design principles CS 564 [Fall 2016] - Paris Koutris 4

5 ENTITIES & ATTRIBUTES Entity an object distinguishable from other object described using a set of attributes Attribute each has an atomic domain: string, integer, Entity set a collection of similar entities CS 564 [Fall 2016] - Paris Koutris 5

6 ENTITIES & ATTRIBUTES price name category name stockprice Product Company Person name address ssn CS 564 [Fall 2016] - Paris Koutris 6

7 RELATIONS A mathematical definition: if A, B are sets, then a relation R is a subset of A x B Example A = {1, 2, 3}, B = {a, b, c, d} R = {(1, a), (1, c), (2, c), (3, b)} A a b c d B CS 564 [Fall 2016] - Paris Koutris 7

8 RELATIONSHIPS price name category name stockprice Product makes Company makes is a subset of Product x Company Person name address ssn CS 564 [Fall 2016] - Paris Koutris 8

9 RELATIONSHIPS price name category name stockprice Product makes Company buys Person employs name address ssn CS 564 [Fall 2016] - Paris Koutris 9

10 MULTIPLICITY OF RELATIONSHIPS one-one A a b c d B many-one A a b c d B many-many A a b c d B CS 564 [Fall 2016] - Paris Koutris 10

11 MULTIPLICITY OF RELATIONSHIPS price name category name stockprice Product makes Company buys Person employs name address ssn CS 564 [Fall 2016] - Paris Koutris 11

12 NOTATION DIFFERENCE We use: The cow book uses (page 33): You should use the notation in the slides! CS 564 [Fall 2016] - Paris Koutris 12

13 MULTI-WAY RELATIONSHIPS How do we model a purchase relation between buyers, products and stores? Product purchase Store Person We can still model this as a mathematical set (ternary relation) CS 564 [Fall 2016] - Paris Koutris 13

14 ARROWS IN MULTI-WAY RELATIONSHIPS What does the arrow mean here? Product purchase Store Person A given person can purchase a given product from at most one store! CS 564 [Fall 2016] - Paris Koutris 14

15 ARROWS IN MULTI-WAY RELATIONSHIPS What about here? Product purchase Store Person A given person can purchase a given product from at most one store AND a given store sells to a given person at most one product CS 564 [Fall 2016] - Paris Koutris 15

16 ARROWS IN MULTI-WAY RELATIONSHIPS How can we say that a given person buys from at most one store? Product purchase Store Person Not possible, we can only approximate! CS 564 [Fall 2016] - Paris Koutris 16

17 ROLES IN RELATIONSHIPS What if we need an entity set twice in a relationship? Product purchase Store Person salesperson Person buyer CS 564 [Fall 2016] - Paris Koutris 17

18 ROLES IN RELATIONSHIPS Label the edges to indicate the roles Collapse the two entity sets into one Product purchase Store salesperson buyer Person CS 564 [Fall 2016] - Paris Koutris 18

19 ATTRIBUTES IN RELATIONSHIPS date Product purchase Store Person CS 564 [Fall 2016] - Paris Koutris 19

20 MULTI-WAY TO BINARY RELATIONSHIPS Product purchase Store productof Product Person Purchase storeof Store Why do we need the arrows? buyerof Person CS 564 [Fall 2016] - Paris Koutris 20

21 RELATIONSHIPS: RECAP Modeled as a mathematical set Binary and multi-way relationships Converting a multi-way one into many binary ones Constraints on the degree of the relationship many-one, one-one, many-many limitations of arrows Attributes of relationships not necessary, but useful CS 564 [Fall 2016] - Paris Koutris 21

22 ADDITIONAL FEATURES CS 564 [Fall 2016] - Paris Koutris 22

23 SUBCLASSES subclass = specialized case = fewer entities = more properties Example: Products Software products Educational products CS 564 [Fall 2016] - Paris Koutris 23

24 SUBCLASSES price name category version Product ISA Attributes are inherited by the subclasses! age group Software Product Educational Product CS 564 [Fall 2016] - Paris Koutris 24

25 CONSTRAINTS constraint = an assertion about the database that must be true at all times part of the database schema central in database design When creating the ER diagram, you need to find as many constraints as possible! CS 564 [Fall 2016] - Paris Koutris 25

26 TYPES OF CONSTRAINTS keys: SSN uniquely identifies a person single-value: a person can have only one father referential integrity: if you work for a company, it must exist in the database domain: age is between 0 and 150 other: e.g. at most 80 students enroll in a class CS 564 [Fall 2016] - Paris Koutris 26

27 WHY DO WE NEED CONSTRAINTS? Give more semantics to the data help us better understand it Prevent wrong data entry Allow us to refer to entities (e.g. using keys) Enable efficient storage and data lookup CS 564 [Fall 2016] - Paris Koutris 27

28 KEY CONSTRAINTS price name category name stockprice Product makes Company buys Person employs name address ssn CS 564 [Fall 2016] - Paris Koutris 28

29 KEY CONSTRAINTS Every entity set must have a key A key can consist of more than one attribute There can be more than one key for an entity set one key will be designated as primary key No formal way to specify multiple keys in an ER diagram CS 564 [Fall 2016] - Paris Koutris 29

30 SINGLE-VALUE CONSTRAINTS An entity may have at most one value for a given attribute or relationship an attribute of an entity set has a single value a many-one relation implies a single value constraint CS 564 [Fall 2016] - Paris Koutris 30

31 REFERENTIAL INTEGRITY CONSTRAINT A relationship has one value and the value must exist Product makes Company CS 564 [Fall 2016] - Paris Koutris 31

32 WEAK ENTITY SETS Entity sets are weak when their key attributes come from other classes to which they are related Team affiliation University number sport name entities of an entity set need help to identify them uniquely! CS 564 [Fall 2016] - Paris Koutris 32

33 DESIGN PRINCIPLES CS 564 [Fall 2016] - Paris Koutris 33

34 1. BE FAITHFUL TO THE APP! What is wrong here? Product purchase Person Country president Person CS 564 [Fall 2016] - Paris Koutris 34

35 2. AVOID REDUNDANCY! What is wrong here? date Product purchase Store personname personaddress CS 564 [Fall 2016] - Paris Koutris 35

36 2. AVOID REDUNDANCY! Redundancy occurs when we say the same thing in two different ways Redundancy wastes space and encourages inconsistency The two instances of the same fact may become inconsistent if we change one and forget to change the other CS 564 [Fall 2016] - Paris Koutris 36

37 2. AVOID REDUNDANCY! What is wrong here? address Beer manfby Manufacturer manufacturer name CS 564 [Fall 2016] - Paris Koutris 37

38 3. KEEP IT SIMPLE! What is wrong here? Date date Product purchase Store Person CS 564 [Fall 2016] - Paris Koutris 38

39 4. ATTRIBUTES OVER ENTITIES An entity set should satisfy at least one of the following conditions it is more than the name of something; it has at least one non-key attribute it is the many in a many-one or many-many relationship Beer manfby Manufacturer name What is wrong here? name CS 564 [Fall 2016] - Paris Koutris 39

40 5. DON T OVERUSE WEAK ENTITY SETS Beginning database designers often doubt that anything could be a key by itself They make all entity sets weak, supported by all other entity sets to which they are linked In reality, we create unique IDs for entity sets Examples: SSN, ISBN, CS 564 [Fall 2016] - Paris Koutris 40

41 ER MODEL: RECAP Key concepts: entity, attribute, entity set relation: binary, multi-way relationship roles, attributes on relationships subclasses (ISA) weak entity sets constraints many-one, one-one, many-many keys, single-valued, referential integrity design principles CS 564 [Fall 2016] - Paris Koutris 41

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