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I S 2 2 0 : D a t ab a s e F u n d a m e n t a l s

LECTURE 5:

THE RELATIONAL MODEL

Ref. “Chapter3” from

“Database Systems: A Practical Approach to Design, Implementation and Management.”

Thomas Connolly, Carolyn Begg.

Objectives2

Terminology of relational model.

How tables are used to represent data.

Properties of database relations.

How to identify CK, PK, and FKs.

Meaning of entity integrity and referential integrity.

Purpose and advantages of views.

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Logical Data Model

In this phase, the main objective is to translate the conceptual

data model created in phase 1 into a logical data model of

the data requirements of the enterprise.

The logical (relational) model represents the database as a

collection of relations. Informally, each relation resembles a

table of values or, to some extent, a file of records.

Lecture4

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Relational Model Terminology4

A relation is a table with columns and rows.

Only applies to logical structure of the database, not the physical

structure.

Attribute is a named column of a relation.

Domain is the set of allowable values for one or more

attributes.

Relational Model Terminology5

Tuple is a row of a relation.

Degree is the number of attributes in a relation.

Cardinality is the number of tuples in a relation.

Relational Database is a collection of normalized relations

with distinct relation names.

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Instances of Branch and Staff Relations

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Examples of Attribute Domains7

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Alternative Terminology for Relational Model

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Database Relations9

Relation schema

Named relation defined by a set of attribute and

domain name pairs.

Relational database schema

Set of relation schemas, each with a distinct name.

Properties of Relations10

Relation name is distinct from all other relation names in relational schema.

Each cell of relation contains exactly one atomic (single) value.

Each attribute has a distinct name.

Values of an attribute are all from the same domain.

Each tuple is distinct; there are no duplicate tuples.

Order of attributes has no significance.

Order of tuples has no significance.

Relational Keys11

Candidate key (CK) An attribute, or set of attributes, that uniquely identifies a tuple, and no

proper subset is a CK within the relation.

Primary Key Candidate key selected to identify tuples uniquely within relation.

Alternate Keys Candidate keys that are not selected to be primary key.

Foreign Key Attribute, or set of attributes, within one relation that matches candidate key

of some (possibly same) relation.

Representing Relational Database Schemas

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Branch (branchNo, street, city, postcode)

Staff (staffNo, fName, lName, position, sex, DOB, salary, branchNo)

Foreign Key1 branchNo references Branch(branchNo)

PropertyForRent (propertyNo, street, city, postcode, type, rooms, rent,

ownerNo, staffNo,branchNo)

Foreign Key1 staffNo references Staff(staffNo)

Foreign Key2 branchNo references Branch(branchNo)

Client (clientNo, fName, lName, telNo, prefType, maxRent)

Another way for specifying PK:

Branch (branchNo, street, city, postcode)

OR

Branch (branchNo, street, city, postcode)

Primary Key branchNo

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Relational data Model comprising three components:

1. a structural part, consisting of a set of rules according to

which databases can be constructed. (which we discussed in

the previous sections and in the previous lecture: cardinality,

multiplicity and participation).

2. a manipulative part, defining the types of operation that

are allowed on the data (this includes the operations that are

used for updating or retrieving data from the database and

for changing the structure of the database);

3. a set of integrity constraints, which ensures that the data is

accurate.

Integrity Constraints14

Null

Represents value for an attribute that is currently unknown or

not applicable for tuple.

Deals with incomplete or exceptional data.

Represents the absence of a value and is not the same as zero

or spaces, which are values.

Integrity Constraints15

Entity Integrity

State that in a base relation, no attribute of a primary keycan be null.

Referential Integrity

If foreign key exists in a relation, either foreign key valuemust match a candidate key value of some tuple in its homerelation or foreign key value must be wholly null.

Integrity Constraints16

General Constraints

Additional rules specified by users or database

administrators that define or constrain some aspect of the

enterprise.

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Views17

Base Relation

A named relation corresponding to an entity in the conceptualschema, whose tuples are physically stored in database.

View

The dynamic result of one or more relational operationsoperating on base relations to produce another relation.

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Views18

A virtual relation that does not necessarily actually exist inthe database but is produced upon request, at time ofrequest.

Contents of a view are defined as a query on one or morebase relations.

Views are dynamic, meaning that changes made to baserelations that affect view attributes are immediately reflectedin the view.

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Purpose of Views19

Provides powerful and flexible security mechanism by hidingparts of database from certain users.

Permits users to access data in a customized way, so that samedata can be seen by different users in different ways, at sametime.

Can simplify complex operations on base relations.

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