Copyright (c) 2014 Pearson Education, Inc. Introduction to Databases

Copyright (c) 2014 Pearson Education, Inc. INTRODUCTION  Entity-relationship (ER) modeling - conceptual database modeling technique Enables the structuring and organizing of the requirements collection process Provides a way to graphically represent the requirements  ER diagram (ERD) - the result of ER modeling Serves as a blueprint for the database Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 2

Copyright (c) 2014 Pearson Education, Inc. ENTITIES  Entities - constructs that represent what the database keeps track of The basic building blocks of an ER diagram Represent various real world notions, such as people, places, objects, events, items, and other concepts Within one ERD each entity must have a different name Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 3

Copyright (c) 2014 Pearson Education, Inc. ENTITIES Two entities Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 4

Copyright (c) 2014 Pearson Education, Inc. ENTITIES  Entity instances (entity members) - occurrences of an entity Entities themselves are depicted in the ER diagrams while entity instances are not Entity instances are eventually recorded in the database that is created based on the ER diagram Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 5

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES  Attribute - depiction of a characteristic of an entity Represents the details that will be recorded for each entity instance Within one entity, each attribute must have a different name  Unique Attribute - attribute whose value is different for each entity instance Every regular entity must have at least one unique attribute Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 6

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES An entity with attributes Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 7

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS  Relationship - ER modeling construct depicting how entities are related Within an ER diagram, each entity must be related to at least one other entity via a relationship Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 8

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS  Cardinality constraints - depict how many instances of one entity can be associated with instances of another entity Maximum cardinality o One (represented by a straight bar: I) o Many (represented by a crow’s foot symbol) Minimum cardinality (participation) o Optional (represented by a circular symbol: 0) o Mandatory (represented by a straight bar: I) Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 9

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS A relationship between two entities Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 10

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS Four possible cardinality constraints Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 11

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS Several possible versions of the relationship ReportsTo Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 12

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS  Types of Relationships (maximum cardinality-wise) One-to-one relationship (1:1) One-to-many relationship (1:M) Many-to-many relationship (M:N) Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 13

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS Three types of relationships (maximum cardinality-wise) Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 14

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS A 1:M Relationship Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 15 A M:N Relationship A 1:1 Relationship

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS  Relationship instances - occurrences of a relationship Occur when an instance of one entity is related to an instance of another entity via a relationship Relationship themselves are depicted in the ER diagrams while relationship instances are not Relationship instances are eventually recorded in the database that is created based on the ER diagram Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 16

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS A relationship and its instances Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 17

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS  Relationship attributes In some cases M:N relationships can actually have attributes of their own Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 18

Copyright (c) 2014 Pearson Education, Inc. TERNARY RELATIONSHIP Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 19

Copyright (c) 2014 Pearson Education, Inc. RELATIONSHIPS Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 20 Relationship attributes Example

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES  Composite attribute – attribute that is composed of several attributes Not an additional attribute of an entity Its purpose is to indicate a situation in which a collection of attributes has an additional meaning, besides the individual meanings of each attribute Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 21

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES An entity with a composite attribute Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 22

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES  Composite unique attribute – attribute that is composed of several attributes and whose value is different for each entity instance Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 23

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES An entity with a unique composite attribute Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 24

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES  Multiple unique attributes (candidate keys) - when an entity has more than one unique attribute each unique attribute is also called a candidate key Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 25

Copyright (c) 2014 Pearson Education, Inc. ATTRIBUTES An entity with multiple unique attributes (candidate keys) Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 26

Copyright (c) 2014 Pearson Education, Inc.  Task from E-book:  What is a weak entity? Provide an example. Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 27