Steps in designing a database. Steps in picture. Entity-Relationship Model. Entity Set. Attributes. The Entity-Relationship Data Model

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1 Steps in designing a database The Entity-Relationship Data Model Chapter 2 (Database Design) Analysis: What information needs to be stored? What are the relationships between different components of the stored information? What is the suitable database structure (or schema)? Design the database structure (using a database design language or notation suitable for expressing design) Implementation in DBMS once committed to the design 1 2 Ideas/ Problems Steps in picture Database Structure E/R diagram Relational model ODL, ORDL Database Design DBMS Analysis Design Implementation Entity-Relationship Model Traditional & popular Graphical representation Three types of elements Entity set (rectangle) Attributes (oval) Relationship (diamond) 3 4 Entity Set Collection of similar objects Similar to a class in the sense of OOP Entity vs. entity set Example: Database about movies, their stars, the studio that producing them, and other aspects of movies. A movie is an entity Collection of movies is an entity set Other entities in this db:? Attributes Entity set has associated attributes Each attribute represents a property of entities belonging to the entity set Example: The entity set has the associated attribute Title each movie has a Assumption: attributes are atomic values with two components (e.g. number, street) cannot be used as an attribute of an entity set 5 6 1

2 Relationship Connections among entity sets Represent a relationship between entity sets If E and F are two entity sets and R connect the two, then R is a binary relation between E and F, mathematically we write R E x F. Example: and are two entity sets and a connection -In between the two the intention: m is related to s means that star s starts in movie m. E/R Diagram A graph representing entity sets, attributes, and relationships. Entity set (rectangle) Attributes (oval) Relationship (diamond) 7 8 A reading of the previous diagram _in 9 Three entity sets: [,,,] [,] [,] Two relationships (m:n) _in(,) (m:1) (,) 10 Instance of an E/R diagram Concrete data of a database, whose structure is represented by a E/R diagram, creates a database instance In an instance For each entity set: a particular finite set of entities, each has values for each attribute For each relationship connecting the entities E 1, E 2,,E n a set of tuples (e 1,e 2,,e n ) each e i is an entity in the entity set of E i Where are the attributes? 11 Example A possible database instance of the previous E/R diagram (Note: The info is inserted for illustrative purpose only it needs not be true in real life) Basic instinct Basic instinct Drama Mystery Sharon Stone Sharon Stone Arnold Schwarzenegger _in Sharon Stone Arnold Schwarzenegger Basic instinct a1 a2 Universal Universal Universal Dream World 12 2

3 A little of math before continuing E, F are two sets R E x F: R is a binary relation from E to F R is a set whose member is a pair (e,f) where e is a member of E and f is a member of F R could be empty, could be equal the Cartesian product of E and F R is many to one relation if for each e in E there is at most one element (e,f) in R R is one to one: many to one from E to F and many to one from F to E R is many to many: not many to one from E to F and not many to one from F to E many-one one-one 13 one-many many-many 14 Multiplicity of Binary E/R Relationship R is a binary relation from E to F One to many One to one Many to many Representing using arrow in the connection between entity set and relationship Arrow entering an entity set represents the one in the above description 15 _in many-many many-one 16 Multiway Relationships Easy to handle in E/R diagram: connect all related entity sets with the relationship (Note: weakening the relationship (movie,studio)) Roles in Relationship An entity set can appear more than one times in a relationship E/R diagram: labeling the arc connecting the entity set and the relationship Sequel Sequel_of relationship: (studio,star,movie) 17 Original 18 3

4 Another example Attributes on Relationships of star Producing studio 19 How to record the salary of a star in a contract??? salary? 20 Attributes on Relationships How to record the salary of a star in a contract? Ans: attribute of! salary 21 * salary * The complete diagram of the relationship. Can we live without *? 22 Salaries Eliminating Attributes on Relationship Introducing a new entity set whose attributes are the attributes attached to the relationship Connecting the new entity set to the relationship, with the arrow pointed to the new entity set salary

5 Converting Multiway Relationship to Binary Relationship Useful when the language for defining the database structure does not allow multiway relationship. Simple: Introducing a new entity set representing the relationship Introducing binary relation between the new entity set and the old ones which are connecting to the relationship 25 Movie_of of star Star_of of star Producing studio Producing studio Before After 26 Subclasses in the E/R Model Subclass: common in real-life Represented by the isa relationship isa is one-one relationship In E/R diagram: draw as a triangle no arrows into the entity sets Voices Cartoons isa isa Murder Mysteries weapon 27 At home: read example 2.10 and Design Principles Faithfulness Avoiding redundancy Simple Select the right relationships Select the right kind of element Faithfulness Entity sets and attributes should reflect reality. Relationships are created only if they make sense given what we know about the domain/application. Example: _in relationship should be many-many Teaches relationship between Courses and Instructors? What? How?

6 No Redundancy Motto: Stored everything only one! Space reason (less serious) Consistency (serious) Example: Add an attribute studioname to the entity set while having the relationship between and consequences: More space (obvious: studio stored twice) Change in ownership of a movie Ł change in the relationship and the entity set Simplicity Do only whatever is necessary! Example: is better than Represents Holdings Right Relationships Should we represent every possible relationships? NO: due to space & redundancy requirements Ł get only the necessarily one How? Consider the assumptions, identify those that cannot be deduced from or represented by others 33 _in Assumption: a star can plays in a movie if there is a contract involving the star and the movie _in is redundant 34 _in Works_for Right Kind of Element Attribute vs. Entity set/relationship Example: in the examples so far: attribute better as an entity with attributes such as street, number, zip code, state Question: Would it make sense to have a relationship Works_for? Depending

7 Example: What happens if we replace by its two attributes? redundancy, losing information (if a studio does not own a movie its is lost) _in 37 When to use attribute for entity set E? All relationships connecting to E must have an arrow entering E. Attributes for E must collectively identify an entity. If there are more than one attributes then they must not depend on each other. No relationship involves E more than one. 38 Converting from Entity sets to Attributes Converting from Entity sets to Attributes E R F E R F A B A B F F A R F A B 39 B F 40 7