Entity-Relationship Models 2. Connolly/Begg Chapter 11

Transcription

1 Entity-Relationship Models 2 Connolly/Begg Chapter 11 1

2 Overview Odd Bits Recursive relationships Attributes Primary/Foreign keys Strong/Weak entities Problems Connection Traps Building a logical model Converting the ER to the Relational Data Model 2 2

3 Recursive Relationships A relationship type where the same entity type participates more than once in different roles. Connolly/Begg supervises Staff A member of staff supervises many members of staff. The entity type staff has two roles in this relationship: supervisor and supervisee 3 3

4 Attributes Definition A property of an entity or a relationship type Connolly/Begg Each entity type has a set of properties called attributes: Staff has attributes: number name salary Staff number name salary Attributes can be shown on the model diagram 4 4

5 Strong Entities Definition An entity type that is not existence-dependent on some other entity type Connolly/Begg A strong entity has its own existence, independent of all other entities. The staff entity does not depend on other entities. A strong entity is uniquely identifiable using a primary key. Same as a relation. 5 5

6 Weak Entities Definition An entity type that is existence dependent on some other entity type. Connolly/Begg A weak entity can only exist when some other entity exists also. A Job Preference entity type can only exist when a staff entity type exists. Staff Job Preference A staff member has a recorded job preference. 6 6

7 Weak Entities A weak entity type can only be identified through another entity type. It must have a primary key that includes the primary key from another entity type. Staff Job Preference The primary key of the Job Preference entity type must include the primary key for Staff 7 7

8 Overview Odd Bits Recursive relationships Attributes Primary/Foreign keys Strong/Weak entities Problems Connection Traps Building a logical model Converting the ER to the Relational Data Model 8 8

9 Problems with ER Models Connection Traps Problems occurring in relationships between entities Missing information Normally, poorly understood relationships Two types Fan Traps Chasm Traps 9 9

10 Problem - Fan Traps When the pathway between entity types is ambiguous Customer Area Depot An area has many customers. An area has many depots. To which depot has customer C1 been assigned? C1 C2 C3 C4 A1 A2 D1 D2 D3 D

11 Solution - Fan Traps Reorganise the relationships Area Depot Customer An area has many depots. A depot has many customers. To which depot has customer C1 been assigned? A1 A2 D1 D2 D3 D4 C1 C2 C3 C

12 Problem - Chasm Traps Relationship is suggested between entities but the model does not contain the pathway. Depot Order Product The model suggests that a depot stocks a product. What if a product has never been ordered? D1 D2 O1 O2 O3 O4 D1 D2 D3 D4 We cannot find out if a depot stocks a product that has not been ordered

13 Solution - Chasm Traps Create a new relationships Depot Order Product stocks If a product has not been ordered then the fact that a depot stocks a product can still be represented

14 Overview Odd Bits Recursive relationships Attributes Primary/Foreign keys Strong/Weak entities Problems Connection Traps Building a logical model Converting the ER to the Relational Data Model 14 14

15 Building the Logical Model ER Model Conceptual Model Abstract, high level Relational Data Model Logical Model DBMS oriented model An ER model can be converted to a relational data model 15 15

16 Step 1 - M:N Relationships Relational Data Model cannot represent manyto-many relationships directly A foreign key relationship is a one-to-many relationship

17 Step 1 - M:N Relationships Resolve all M:N relationship types to two 1:M relationship types Depot Product Depot Stocks Product A depot has many stocks, a stock is for one product A product has many stocks, a stock is for one depot 17 17

18 Step 2 : Entities Convert entities to relations Create a primary key for each entity Add other attributes to each entity Depot Stocks Product Depot(dno, name, opening-hours) Product(pno, description, price) Stock(dno, pno) The M:N relationships, stock, has a primary key that includes the primary keys from both sides of the relationship

19 Step 3: Relationships Convert 1:M relationships to foreign keys Take the primary key from the one side of the relationship and make if a foreign key in the many side of the relationship. Area Depot Customer Area(ano, location) Depot(dno, name, opening-hours, ano*) Customer(cno, name, street, postcode, dno*) 19 19