Relational Model

Relational Model

• First introduced in 1970 by E.F. Codd at IBM.

• Two early research projects on relational model:

• IBM System R (prototype RDBMS)• UC Berkeley’s Ingres (academic RDBMS)

A tabular representation of data.A major strength of the relational model:

supports simple, powerful querying of data.

Relational Model

• Relational model -basic concepts

• Enforcing Data Integrity Constraints

• Structure of Relational Databases

• Fundamental Relational-Algebra-Operations

• Extended Relational-Algebra-Operations

• Null Values

• Modification of the Database

• Relational Calculus

• Assertion and Triggers

• Introduction on views

• Codd's Rules

Domains, Attributes, Tuples and Relations:

• Relational database: a set of relations.

• Relation: made up of two parts:• Instance : a table, with rows and columns.

#Rows = cardinality, #fields = degree / arity.• Schema : Description of data at some abstraction

level which specifies name of relation, plus name and type of each column.

• Students (sid: string, name: string,

login: string, age: integer,

gpa: real )

Example of a Relation

sid name login age gpa

53666 Jones jones@cs 18 3.4

53688 Smith smith@eecs 18 3.2

53650 Smith smith@math 19 3.8

oCan think of a relation as a set of rows or tuples (i.e., all rows are distinct).

oDo all columns in a relation instance have to be distinct?

Attribute Types:

Each attribute of a relation has a name The set of allowed values for each attribute is called

the domain of the attribute Attribute values are (normally) required to be atomic;

that is, indivisible• E.g. the value of an attribute can be an account

number, but cannot be a set of account numbers.

Domain is said to be atomic if all its members are atomic

The special value null is a member of every domain but null value causes complications in the definition of many operations.

Relation Schema:

Schema of a relation consists of• Attribute definitions • name• type/domain

Integrity constraints: Restriction on state (or of sequence of states) of a database

• The values of a particular attribute in all tuples are unique (structural).• The values of a particular attribute in all

tuples are greater than zero (Sementic).

Relation Instance:

The current values (relation instance) of a relation are specified by a table.

An element t of r is a tuple, represented by a row in a table

Order of tuples is irrelevant (tuples may be stored in an arbitrary order)

JonesSmithCurryLindsay

customer_name

MainNorthNorthPark

customer_street

HarrisonRyeRyePittsfield

customer_city

customer

attributes(or columns)

tuples(or rows)

Database:

• A database consists of multiple relations

• Information about an enterprise is broken up into parts, with each relation storing one part of the information

• E.g.

account : information about accounts depositor : which customer owns which account customer : information about customers

Relations:

depositor

customer

Schema Diagram

Why Split Information AcrossRelations?

• Storing all information as a single relation such as bank (account_number, balance, customer_name, ..)results in

• Repetition of information

• e.g.,if two customers own an account

• The need for null values

• e.g., to represent a customer without an account

• Normalization theory deals with how to design relational schemas.

Integrity constraints

• IC: condition that must be true for any instance of the database; e.g., domain constraints.• ICs are specified when schema is defined.• ICs are checked when relations are modified.

• A legal instance of a relation is one that satisfies all specified ICs. • DBMS should not allow illegal instances.

• Avoids data entry errors, too.

• Automatically checked by DBMS.

• Enforces enterprise rules.

Key constraint

• A relation instance s of S (relational schema)

satisfies the key constraint iff at most one row in s can contain a particular set of values, a1,…,an, for the attributes A1,…,An (sequence of attributes ).

• Key• Set of attributes mentioned in a key constraint

• e.g., Id in Student relation.

Keys

• Let K R

• K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(R)

• In real life, an attribute such as customer_id would be used instead of customer_name to uniquely identify customers.

Keys (Cont.)

• K is a candidate key if K is minimalExample: {customer_name} is a candidate key for Customer, since it is a superkey and no subset of it is a superkey.

• Primary key: a candidate key chosen as the principal means of identifying tuples within a relation

• Should choose an attribute whose value never, or very rarely, changes.

• E.g. email address is unique, but may change.

• Possibly many candidate keys, one of which is chosen as the primary key.

Foreign Keys

• A relation schema may have an attribute that corresponds to the primary key of another relation. The attribute is called a foreign key.

• E.g. customer_name and account_number attributes of depositor are foreign keys to customer and account respectively.

• Only values occurring in the primary key attribute of the referenced relation may occur in the foreign key attribute of the referencing relation.

Schema Diagram

Example:

sid name login age gpa

53666 Jones jones@cs 18 3.453688 Smith smith@eecs 18 3.253650 Smith smith@math 19 3.8

sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 B

Stud_

Views:

• A view is just a relation, but we store a definition, rather than a set of tuples.

CREATE VIEW YoungActiveStudents (name, grade)AS SELECT S.name, E.gradeFROM Students S, Enrolled EWHERE S.sid = E.sid and S.age<21

Query Languages

• Language in which user requests information from the database.

• Categories of languages• Procedural• Non-procedural, or declarative

• “Pure” languages:• Relational algebra• Tuple relational calculus• Domain relational calculus

• Pure languages form underlying basis of query languages that people use.