Database Management System by

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1 Database Management System by Meri Dedania Assistant Professor MCA department Atmiya Institute of Technology & Science Yogidham Gurukul Rajkot

2 Chapter -6 Entity Relationship Data Model

3 6.2 The Entity-Relationship Model Why E-R data model is introduced? Database Design of larger Relational Database Easy to Understand The basic Notions of E R data model 1) Entity Set : Table 2) Relationship Set: 3) Attributes: Column

4 Entity Set What is Entity? An entity is a thing or object in the real world that is distinguishable from all other objects. An entity has some set of properties and value for some set of properties may uniquely identifies. What is entity set? An entity set is set of entities of the same type that share the same properties or attributes. The individual entities that represents a set are said to be the extension of entity set.

5 Types of Entity(Concrete & Abstract) Entity may be concrete(tangible) like a person or a book or customer or employee. Or it may be abstract(intangible) like a holiday, transaction or loan. What are Attributes? An entity is represented by a set of attributes. Attributes are descriptive properties hold by each member of entity set. Values of entity for each of its attributes.

7 Examples for each term A person who are customer of particular bank is entity of entity set CUSTOMER. So CUSTOMER is entity set and all persons who are customers of bank are entities. Some Properties or Attributes Which are shared by CUSTOMER are cust_id, cust_name, c_street, c_city. CUSTOMER is Tangible Entity. One another example is LOAN entity set contains information for the loan given by particular bank to Customer. LOAN has Attributes like loan_num, amount. Loan is intangible entity.

8 The value of Properties like 1001 for cust_id, Hayes for cust_name, Main for c_street, Harrison for c_city Entity set do not need to be disjoint. Suppose one Entity set EMPLOYEE for information of employee are in particular bank. Who is also Customer of bank. That person will entity of CUSTOMER & EMPLOYEE both, or none. The cust_id & loan_num attributes are used to identify customer and loan uniquely respectively.

9 Attributes What is Attributes? Attributes are descriptive properties which hold by each member of entity set. Domain or value set The set of permitted value called Domain or value set of that attribute. Example of Domain Like Domain of cust_name is set of string of a certain length. Similarly, loan_num is set of string of the form L-n where n is positive integer.

10 An attribute of an entity set is a function that maps from the entity set into a domain. Since an entity set have several attributes, each entity can be described by a set of pairs, one pair for each attributes of the entity set. A particular CUSTOMER entity may be describe by the set (cust_id, 1001), (cust_name, Hayes),(c_street, Main ), (c_city, Harrison).

11 Types of Attributes : 1) Simple and composite attributes Which attribute does not divide into subpart is called simple attributes. For example Cust_name in CUSTOMER does not have subpart. Which Attributes divided into subparts is called Composite Attributes. Cust_name divided into First_name, Middle_name, Last_name. Also we can substitute the composite attribute like address for C_street, C_city. Note that composite attribute may appear as a hierarchy. In Composite attribute like Address is divided into street_number, street_name, and appartment_number.

13 2) Single valued and multivalued Attributes. Which attributes have single value for particular entity is called single valued attribute. Which attributes have multiple value for particular entity is called multi valued attribute. For example in LOAN, the loan_num attribute for a specific loan entity refers to only one loan number. Assume that an entity set EMPLOYEE has attribute like phone_number. An employee may have zero, one or more phone number. Another example of dependant_name of an EMPLOYEE would have zero, one or more dependant. The upper bound and lower bound of multi value can be assign in number.

14 3) Derived Attribute The value of this kind of attribute can be derived from the values of other related attributes or entities. For example, the customer entity set has attribute age that indicates the customer s age. If CUSTOMER has also attribute like Date_of_birth. We can calculate age from the date_of_birth and the current date. So age is derived attribute and date_of_birth is called based or stored attribute. The value of a derived attribute not stored but is computed when required.

15 Relationship Set What is relationship? A relationship is an association among several entities. For example, two entity sets CUSTOMER and LOAN. The relationship set borrower to denote the association between CUSTOMER and bank loans that the customers have. The association between entity sets is referred to as participation. The function that an entity plays in a relationship is called that entity s role. The same entity set participates in a relationship set more then once, in different roles is called recursive relationship set.

17 A relationship may also have attributes called descriptive attributes.

18 The relationship set borrower provide an example of a binary relationship set. Binary relationship set involves two entity sets. The number of entity sets that participate in a relationship set is also the degree of the relationship set. For example, a binary relationship set is of degree 2.

6.3 - Constraints E R enterprise schema may define certain constraints which data value must satisfy. Here we examine mapping cardinalities, key constraints and participation constraints. 1) 6.3.1- Mapping Cardinalities : It represents number of entities to which another entity can be associated via a relationship set.

19 6.3 - Constraints E R enterprise schema may define certain constraints which data value must satisfy. Here we examine mapping cardinalities, key constraints and participation constraints. 1) Mapping Cardinalities : It represents number of entities to which another entity can be associated via a relationship set. It is very useful to describe binary relationship sets. The mapping cardinality must be one of the following.

20 One to One : Any entity set A is associated with at most one entity in B. And an entity in B is associated at most one entity in A.

21 One to Many : An entity in A is associated with any(zero or more) number of entities in B. An entity in B, however, can be associated with at most one entity in A.

22 Many to One : An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number(zero or more)of entity in A.

23 Many to many : An entity in A is associated with any number (zero or more) of entities in B. And an entity in B is associated with any number (zero or more) of entities in A.

24 2) Keys Entity Sets Relationship Sets

25 3) Participation Constraints If every entity in entity set E participates in at least one relationship in R. then relationship set R is said to be total. If some entity in entity set E participates in relationship set R then R is said to be Partial.

26 6.4 Entity Relationship Diagrams E R diagram can express the overall logical structure of a database graphically. Diagram consists of the following major components.

Design Process Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to

Design Process Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML A database can be modeled