DATA MODELLING AND ENTITY-RELATIONSHIP DIAGRAM

Transcription

1 DATA MODELLING AND ENTITY-RELATIONSHIP DIAGRAM EGCO321 DATABASE SYSTEMS KANAT POOLSAWASD DEPARTMENT OF COMPUTER ENGINEERING MAHIDOL UNIVERSITY

2 DATABASE MODEL Collection of logical constructs used to represent data structure and relationships within the database Conceptual models: logical nature of data representation Implementation models: emphasis on how the data are represented in the database

3 ENTITIES AND ATTRIBUTES (1) The basic object that the ER Model represents is an entity, which is a thing in the real world with an independent existence. Each entity has attributes the particular properties that describe it.

4 ENTITIES AND ATTRIBUTES (2) Example: Two entities, employee e 1 and company c 1, and their attributes.

5 ENTITIES AND ATTRIBUTES (3) Composite and Simple (Atomic) Attributes.

6 ENTITY TYPES AND ENTITY SETS (1) A database usually contains groups of entities that are similar. For example, a company employing hundreds of employees may want to store similar information concerning each of the employees. These employee entities share the same attributes, but each entity has its own value(s) for each attribute. An entity type defines a collection (or set) of entities that have the same attributes. The entity set is usually referred to using the same name as the entity type.

7 ENTITY TYPES AND ENTITY SETS (2) Two entity types, employee and company.

8 ENTITY TYPES AND ENTITY SETS (3) Value Sets (Domains) of Attributes. Each simple attribute of an entity type is associated with a value set (or domain of values) In previous figure (previous page), if the range of ages allowed for employees is between 16 to 70, we can specify the value set of the Age attribute of EMPLOYEE to be the set of integer number between 16 to 70. Value sets are typically specified using the basic data types available in most programming languages, such as integer, string, or boolean.

9 CONCEPTUAL DESIGN (1) Design of the COMPANY database

10 CONCEPTUAL DESIGN (2) Relationship Types and Relationship Sets. Some instances in the WORKS_FOR relationship sets, which represents a relationship type WORKS_FOR between EMPLOYEE and DEPARTMENT

11 CONCEPTUAL DESIGN (3) Relationship Degree, Role Names, and Recursive Relationships Some relationship instances in the SUPPLY ternary relationship set.

12 CONCEPTUAL DESIGN (4) Constrains on Relationship Types Relationship types usually have certain constraints that limit the possible combinations of entities that may participate in the corresponding relationship set. We can distinguish two main types of relationship constraints: Cardinality Ratio Participation

13 CONCEPTUAL DESIGN (5) Cardinality Ratio for Binary Relationships One-to-one (1:1) One-to-many (1:M) Many-to-many (M:N)

14 CONCEPTUAL DESIGN (6) A 1:1 relationship, MANAGES A M:N relationship, WORKS_ON

15 CONCEPTUAL DESIGN (7) Participation Constraints (Connectivity) specifies whether the existence of an entity depends on its being related to another entity via the relationship type. This constraint specifies the minimum number of relationship instances that each entity can participate in, sometime called the minimum cardinality constraint.

16 CONCEPTUAL DESIGN (8) Attributes of Relationship Types Relationship types can also have attributes, similar to those of entity types. For 1:1 or 1:N relationship types can be migrated to one of the participating entity types. For M:N relationship types, some attributes may be determined by the combination of participating entities in a relationship instance, not by any single entity. Such attributes must be specified as relationship attributes.

17 WEAK ENTITY TYPES Entity types that do not have key attributes of their own are called weak entity. In contrast, regular entity types that do have a key attribute are called strong entity type. We call the relationship type that relates a weak entity type to its owner the identifying relationship (or strong relationship) of the weak entity.

18 ENTITY-RELATIONSHIP DIAGRAM

19 ATTRIBUTES

20 ERD WITH CARDINALITY

21 OPTIONAL RELATIONSHIP

22 EXAMPLE

23 RELATIONSHIP STRENGTH (1) Existence Dependence Child entity existence dependent on existence of related parent entity. Existence-independent entities can exist apart from related entities. Weak (Non-Identifying) Relationship Entity is existence-independent of other entities. Strong (Identifying) Relationship Child entity is existence-dependent on parent.

24 RELATIONSHIP STRENGTH (2)

25 STRONG RELATIONSHIP AND WEAK ENTITY An entity type that borrows all or part of its primary key from another entity type. Identifying relationships indicate the entity types that supply components of the borrowed primary key.

26 WEAK ENTITIES (1)

27 WEAK ENTITIES (2)

28 SELF REFERENCING (UNARY)

29 M-WAY (TERNARY) RELATIONSHIP

30 RELATIONSHIP EQUIVALENCE A M-N relationship can be replaced by an associative entity type and two identifying 1-M relationships. In most cases the choice between a M-N relationship and the associative entity type is personal preference.

31 M-N RELATIONSHIP TRANSFORMED INTO 1-M RELATIONSHIP

32 GENERALISATION HIERARCHY

33 DISJOINTNESS AND COMPLETENESS

34 SUMMARY OF CROW S FOOT NOTATION

35 SUMMARY OF CHEN NOTATION

36 HOMEWORK 1 Draw an entity-relationship diagram and list all attributes including the primary key for each entity type, for a database suitable for showing the family tree. (fatherhood, motherhood, brotherhood, sisterhood, cousins, nephews, nieces, and grandparents) Remember that it is possible to reproduce without marriage and that marriage does not imply reproduction.