Relational Model - An Introduction

Relational Model - An Introduction

• 1.
  Relational Model Rajeev Srivastava
• 2.
  ToC • Relation • Tuples •Cardinality • Attribute • Degree/Arity • Domains • Relation vs. Table 2 • Candidate Key • Super Key • Primary Key • Composite Key • Foreign Key • Integrity Rules [email protected]
• 3.
  Relation In Relational Modeldata is modelled in form of Relations represented by tabular structure. Consider the relation EMPLOYEE represented by the following table: 3 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode [email protected]
• 4.
  Tuples in aRelation A relation is a set of tuples; each row here is a tuple : 4 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode 1 2 3 4 5 7 6 [email protected]
• 5.
  Cardinality of aRelation No of Tuples in a Relation at a point in time. Cardinality = 7 5 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode 1 2 3 4 5 7 6 [email protected]
• 6.
  Attribute in aRelation An attribute represents a quality/information about an entity. A tuple consists of Attribute values. 6 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode [email protected]
• 7.
  Degree (=Arity) ofa Relation A degree or arity of a Relation is the number of attributes in it. Degree = 8 7 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode [email protected]
• 8.
  Domains Each attribute hasa domain associated with it. Attribute values in a relation are restricted to the values from its domain. 8 EmpCode Name Desig Grade JoinDate BasicSalary Gender DeptCode DEPT ACCO PURC COUR DESIG PE TO STO [email protected]
• 9.
  Consider the Employeerelation defined as : create table EMPLOYEE( EmpCode integer(4), Name char(30), Desig char(4), Grade integer(4), JoinDate date, Basic integer(7), Gender char(1), DeptCode char(4) ) [email protected]
• 10.
  Domains of Attributesof Employee Relation : EmpCode set of all 4-digit numbers Name set of all 30-alpha characters Desig set of all designation codes Grade set of all grade values JoinDate set of all dates (in a given range) Basic set of all possible values for basic Gender set {'M','F‘, ‘T’} DeptCode set of all dept codes [email protected]
• 11.
  A Relation maybe represented as a Table where 11 Relation Table Tuple Row/Record Attribute Column Degree/Arity No of Columns in the table Cardinality No or Rows in the table Domain Pool of acceptable values for a column Primary Key Unique Identifier [email protected]
• 12.
  But, a Relationis not a Table, because : • A table has an inherent order for rows; there is no concept of order for tuples in a relation. • A relation must have a primary key; a table need not have an identifier. • The tuples in a relation must be unique; there is no such restriction for tables [email protected]
• 13.
  Relation : Observations •A Relation is a set of tuples. • A Relation is time-variant. • A Relation cannot have duplicate tuples. • Tuples in a Relation are unordered. • Values are Atomic. • Attribute values are of same kind. • A Relation is a subset of the Cartesian Product of a set of domains. [email protected]
• 14.
  Candidate Key &Super Key A set of attributes is said to be Candidate Key if and only if it satisfies the following time-independent properties: Uniqueness property: No two distinct tuples have the same value for the key. Minimality property: None of the attributes of the key can be discarded from the key without destroying the uniqueness property. A Super Key is a Non-Minimal Candidate Key. Its a set of one or more columns in a table for which no two rows can have the exact same values. An Alternate Key is a candidate key that is not the primary key. [email protected]
• 15.
  Candidate Key? create tableEMPLOYEE( EmpCode integer(4), Name char(30), Desig char(4), Grade integer(4), JoinDate date, Basic integer(7), Gender char(1), DeptCode char(4) ) [email protected]
• 16.
  Candidate Key? create tableEMPLOYEE ( EmpCode integer(4), Name char(30), Desig char(4), Grade integer(4), JoinDate date, Basic integer(7), Gender char(1), DeptCode char(4), Email char(100), MobileNo char(16) ) [email protected]
• 17.
  Primary Key is acandidate key that have following two qualities - • Uniquely identifies a tuple in a relation • Must NOT be NULL *Should be selected from candidate keys such that it never/rarely changes. [email protected]
• 18.
  Primary Key? create tableEMPLOYEE ( EmpCode integer(4), Name char(30), Desig char(4), Grade integer(4), JoinDate date, Basic integer(7), Gender char(1), DeptCode char(4), Email char(100), MobileNo char(16) ) [email protected]
• 19.
  Composite Key • Acandidate key with two or more attributes that uniquely identifies the tuple in a Relation. • Also called as compound key Composite Primary Key • A primary key which is a composite key is called as Composite Primary Key. [email protected]
• 20.
  Can we havemore than one primary key in a table? No. We can not. People who are new to RDBMS are often get confused with the restriction of single Primary Key in a table and the concept of Composite Key. To clarify the same - A table can have only one Primary Key. The Primary Key can be defined on a single column or more than one columns. If the Primary Key is defined using more that one columns, it is known as a Composite Key (or Composite Primary Key). Therefore, a Composite Key in a table does not mean that there are more than one Primary Keys in the table. Instead, a Composite Key uses more than one columns to define a (Single) Primary Key. [email protected] 20
• 21.
  Foreign Key • AForeign Key is a set of attributes in one relation whose values are required to match one of the values of the primary key of the same or different relation. • There can be more than one foreign key in a given relation. [email protected] Identify a relation in any system/business, define its Attributes, Domain for each attribute and find out Primary, Key, Foreign Keys, Candidate Keys, Super Key in the relation.
• 22.
  Foreign Key(s)? create tableEMPLOYEE( EmpCode integer(4), Name char(30), Desig char(4), Grade integer(4), JoinDate date, Basic integer(7)), Gender char(1), DeptCode char(4) ) create table DEPT( DeptCode char(4), DeptName char(30), Location char(10) ) [email protected]
• 23.
  Integrity Rules Entity Integrity:Implemented through Primary Key “No Attribute participating in the primary key of a relation may accept null values” Guarantees that each tuple will have a unique identity. Referential Integrity: Implemented through Foreign Key “Values of the foreign key (a) must be either null, or (b) if non-null, must match with the primary key value of some tuple of the `parent’ relation. The reference can be to the same relation” *Foreign Key is also know as Reference/Referential key. [email protected]
• 24.
  Relational Database Operations •SELECTION: This selects some or all of the records in a table. Through WHERE Clause • PROJECTION: This limits columns from a table. Through SELECT Clause • UNION • INTERSECTION • DIFFERENCE • JOIN • CARTESIAN PRODUCT [email protected] 24