Database Management Systems LECTURE NOTES 2

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1 Database Management Systems LECTURE NOTES 2 Relation: A table; Tuple: A row in a table; Attribute: A column in a table Degree: number of attributes; Cardinality: number of tuples Entity and Entity Sets: Entity: An entity is an object or thing in the real world which can be distinguishable from all other objects i.e. it has an independent existence. For examples of entity is each person in an enterprise, each student of an institute, each customer of a banking system, each employee of an organization, etc. Entity Sets:An entity set is a set of entities of the same type that share the same properties or attributes. The set of all students of an institute can be said as an entity set Student, the set of all customers of a banking system can be said as an entity set Customer, for banking system in another way we can say the set of all persons who are customers can be defined as an entity set Customer. Attributes and Attribute Sets: An entity is presented by a set of attributes. Attributes can be defined as the descriptive properties processed by each member of an entity set. Each entity may have its own value for each attribute. Example, possible attributes for the Student entity set are student_roll_no, student_name, student_address, student_dept and student_ph_no. Possible attributes for the Customer entity set are customer_id, customer_name, customer_street, and customer_city. Domain: An attribute of an entity set has a particular value. The set of possible values that a given attribute can have is called its domain. Each entity has a set of permitted values for each of its attributes. Different types of Attributes: An attribute can be categorized by the following attribute types. a) Simple (or atomic): Simple or atomic attribute can be define as attributes that are not divisible into subparts. The term Atomic is used for specifying it cannot be splitted. For example, in student entity set, student_dept, roll_no, dept_no, college_id etc. are the simple attributes, in customer entity set cust_id is the simple attribute. b) Composite attribute: Composite attribute can be defined as the attribute which can be subdivided into several small subparts. Example, name can be divided into three small subparts, a) first_name b) middle_name c) last_name. we can represent this in this form name (first_name, middle_name, last_name)

2 Another example, address, can be subdivided into several small subparts like a) street b) city c) state d) zip_code. We can represent in this form: address (street, city, state, zip_code) c) Component Attribute: If any subpart of a composite attribute can be divided into subparts again then that attribute is known as component attribute For example, street is one subpart of a composite attribute address. Again street can be divided into a) street_number, b) street_name c) apartment_number. So street is known as component attribute. d) Single-valued Attribute: Attributes those have single value, such attributes are known as single-valued attribute. For example roll_no is single-valued for student entity set; age is single-valued for customer entity set etc. e) Multivalued Attributes: Attributes those have more than one values, is called multivalued attributes. A customer may have more than one phone number. So phone_no is the example of multivalued attribute. College_degree of a person may be more than one. So college_degree is another multivalued attribute. Multivalued attribute can be represented using { }. Within this bracket values are to be written. For these values, it is not mandatory to put one or more value. It may have no values. A customer may have no phone number or one phone number or more than one phone number. So this field may be null as per requirement. f) Derived attribute:derived attribute can be defined as an attribute whose value can be derived from another attribute, known as stored or base attribute. Suppose age of a customer is to be calculated from another attribute date_of_birth of that customer. Here, stored or base attribute is date_of_birth and derived attribute is age. The experience of an employee is to be calculated from the date_of_joining of an employee, for this example, derived attribute is experience.

g) NULL values: An attribute takes a null value when an entity does not have a value for it.

Null can also designate that an attribute value is unknown.

Mapping Cardinalities or cardinality ratios: Mapping cardinalities or cardinality ratios can be defined as the number of entities to which another entity can

Mapping cardinalities are most useful in describing binary relationship sets, although they can constitute to the description of relationship sets that

For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following four categories: a) One to one: An entity

3 g) NULL values: An attribute takes a null value when an entity does not have a value for it. The null value may indicate not applicable that is, that value does not exist for the entity. For example, one may have no middle name. Null can also designate that an attribute value is unknown. An unknown value may be either missing (we do not have that information) or not known (we do not know whether or not the value actually exists). Mapping Cardinalities or cardinality ratios: Mapping cardinalities or cardinality ratios can be defined as the number of entities to which another entity can be associated via a relationship set. i.e., a mapping cardinality is a data constraint that specifies how many entities of an entity can be related to another relationship set. Mapping cardinalities are most useful in describing binary relationship sets, although they can constitute to the description of relationship sets that involve more than two entity sets. For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following four categories: a) One to one: An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A. Only one of the teachers can be head of a department. And one department can have maximum of one head. TEACHER IS HEAD OF DEPARTMENT e.g.: Husband and Wife relation, Automobile and license number b) One to many: An entity in A is associated with any number (Zero or more) of entities in B, and an entity in B can be associated with at most one entity in A.

4 A department can have many teachers DEPARTMENT HAS TEACHER e.g.: Father and children relationship, customer and loan c) Many to one: An entity in A is associated with at most one entity in B and an entity in B can be associated with any number (Zero or more) of entities in A. TEACHER MEMBER OF DEPARTMENT e.g.: goods and supplier, project and manager d) Many to many: An entity in A is associated with any number (Zero or more) of entities in B and an entity in B can be associated with any number (Zero or more) of entities in A.

5 TEACHERS TEACHES STUDENTS e.g.: vendor and client, student registered for course Existence Dependencies If the existence of entity X dependence on the existence of Y, then X is said to be existence dependent on Y. (Y is the dominant and X is the subordinate entity) ACCOUNT LOG TRANSACTION Consider a one-to-many relation from account to transaction. Transactionentity has no meaning unless an account entity is there. If an account entity is deleted, its associated transaction entities must also be deleted. Similarly Repayment entities are existence dependent on Loan entity. Participation: The participation constraints specify whether the existence of an entity depends on its being related to another entity via the relationship type. Participation in a relationship set R by entity set A may be i) Total: It means every entity in A participates in at least 1 relationship in R ii) Partial:It means only some a in A participates in relationship in R EMPLOYEE M WORKS ON N PROJECT Partial Total

6 Every project has at least one employee works on it. But not every employee in the company works in a project Keys: Key: A key can be defined as an attribute or a set of attributes by which we can uniquely identify each entity from each other. Superkey: A Superkey is a set of one or more attributes that taken collectively, allows us to identify uniquely an entity in a relation. Candidate Key: Minimal subsets of superkeys are called candidate keys. Primary Key: Primary key is a candidate key that is chosen by the database designer as the principal means of identifying entities within a relation A key (whether primary, candidate, or super) is the property of entire relation, rather than of the individual entities. The designation of a key represents a constraint in the real-world enterprise being modeled. Foreign Key: Foreign Key is a column or a combination of columns whose values match a Primary Key in a table. A foreign key is used to establish and enforce a link between the data in two tables. For example, consider a database with two tables, a CUSTOMER table that includes all customer data and an ORDER table that includes all customer orders. Suppose that the business requires that each order must refer to a single customer. To reflect this in the database, the primary key (e.g., CUST_ID) in the CUSTOMER table is added to the ORDER table, where it is called a foreign key. Since CUST_ID in the ORDER table uniquely identifies a row of the CUSTOMER table, it says which customer placed the order. CUST_ID A001 A002 A003 A004 CUST_NAME Primary Key A. DAS B. ROY C. SCOTT A. GUPTA ORDER _ID CUST_ID ORDER_AMT Foreign Key ORD_001 A ORD_002 A ORD_003 A ORD_004 A002 50

7 Entity-Relationship Diagram: Entity-Relationship Diagram (E-R Diagram or ERD) is the pictorial or graphical or diagrammatical representation of the logical or conceptual structure of a database system. The following diagrams are used for representing the logical or conceptual structure of the database system. 1 Rectangles Represent entity sets. 2 Ellipses Represent attributes. 3 Diamonds Represent relationship sets. 4 Lines Represents link attributes to entity sets and entity sets to relationship sets. 5 Double ellipses Represent multivalued attributes. 6 Dashed ellipses Denote derived attributes. 7 Double lines Indicate total participation of an entity in a relationship set.

8 Double rectangles Represent weak entity sets. 8 9 Double diamonds Represent weak relationship set. 10 Underlined attribute User_id Denotes primary key. 11 Dotted underlined attribute User_id Denotes discriminator (primary key of weak entity set). Consider one entity set, customer with the following properties (attributes). a) cust_id(primary key) b) cust_name c) cust_addr d) cust_phone e) cust_ f) cust_dob g) cust_age This entity set can be represented in three different ways. 1. Using schema definition 2. Using schema diagram 3. Using E-R diagram 1. Using schema definition customer(cust_id, cust_name, cust_addr, cust_phone, cust_ , cust_dob, cust_age) 2. Using schema diagram: there are two types of representations cust_id cust_name cust_addr cust_phone cust_ cust_dob Cust_age cust_id cust_name cust_addr

9 cust_phone cust_ cust_dob cust_age 3. Using E-R diagram If we want to represent composite attribute, component attribute, and multivalued attribute and derived attribute then diagram will be as like this

10 Degree of relationship type The degree of a relationship type is the number of participating entities type. 1. Unary (Recursive) relationship:this relationship is the relationship that involves only one entity type (same type) but in different roles. Supervisor EMPLOYEE SUPERVISE Supervisee

11 2. Binary Relationship:This relationship has two entity types link together. This is the most common relationship type. EMPLOYEE WORKS ON PROJECT 3. Ternary Relationship:If there are three entity types link together, the relationship is called ternary relationship. A ternary relationship R is a relationship among instances of three different entitiestypes, E1, E2, and E3(R E1 E2 E3). Each instance of the ternary relationship R requires the participation of an instance from each of the entity types E1, E2, and E3. It is sometimes possible (without losing information) to replace higher-order relationship types (ternary and above) by a collection of binary relationship types linking pairs of the original entity types. e.g.: Employee, Project, and Part Employeeworks on project, employee assigned to part, project has parts. Strong Entity Set:A strong entity set is that entity set which must have a primary key. Weak Entity Set: An entity set is said to be weak entity set if it does not have a primary key. i.e. The entity set which does not have sufficient attributes to form a primary key is called as Weak entity set. Consider an entity set Payment which has three attributes: payment_number, payment_date and payment_amount. Although each payment entity is distinct but payment for different

12 loans may share the same payment number. Thus, this entity set does not have a primary key and it is an entity set. Each weak set must be a part of one-to-many relationship set. The existence of a weak entity set depends on the existence of a strong entity set. Thus a weak entity set does not have sufficient attributes to form a primary key. The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence depends, plus the weak entity set s discriminator. Emp_id Emp_name Name DOB EMPLOYEE HAS DEPENDANT Emp_designation Relation It may be of many dependents of many employees with same name and date of birth (DOB). Similarly for Airlines system, flight departure entity has the attributes, date, time and no_of_passengers. But in same date and same time it may be more than one flight are ready for departure. But for which flight we are taking about is not clear until we include the flight number with that entity. Discriminator (or partial key):the discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set on one particular strong entity. The primary key of the strong entity set on which the weak entity set is existence dependent + the weak entity set s discriminator. Payment_number discriminator of the payment entity set Primary key for payment (loan_number, payment_number)

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