Dbms Ch3 Doc

8/3/2019 Dbms Ch3 Doc

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THE RELATIONAL DATABASE

MODEL

The relational database model was developed

by E.F.Codd in 1970.

The relational model is based on three welldefined components:

1. A logical data structure represented by a

relational table where a data is stored.

2. A set of integrity rules to enforce that the

data remains consistent over time.

3. A set of operations that define how the

data are manipulated.

The relational database model allows us to

focus on the logical representation of the

data and relationships rather than the

physical storage details.


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Table

Table plays an important role in relationalmodel of the database.

A table is two-dimensional structure

composed of rows and columns.

It is a logical construct and not a physical

construct.

A table is also called as relation in relational

model terminology used by Codd.

A table is persistent relation, that is, a relation

whose contents can be permanently stored for

future use.

The rows are also called records and columns

are called fields

Characteristics of a Relational Table


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1. A table is perceived as a two- dimensional

structure consisting of rows and columns.

2. Each table row (tuple) within the table

represents a single entity occurrence

within the entity set.

3. Each table column represents an attribute,and each column has distinct name.

4. Each row/column intersection represents a

single data value

5. All values in a column should be of the

same data format. E.g. All integers or all

float

6. Each column has a specific range of

values called as attribute domain.

7. The order of rows and columns is not imp

8. Each table must have an attribute or set of

attributes which uniquely identifies a row


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Keys

A key consists of one or more attributes that

determine other attributes.

Determination in the context of database

If you know the value of an attribute A then

you can get (determine) the value of

attribute B. This means A Determines B.

A B

Functional Dependence

The attribute B is said to be functionally

dependent on A if each value in column A

determines one and only one value in column

B. Stud_Num Stud_Phone

SuperKey


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An attribute or combination of attributes that

uniquely identify each row in a table is called

super key.

Candidate Key

A superkey without redundancies is called a

candidate key. A minimal superkey is acandidate key

Stud_Num

Stud_Num, Stud_Lname

Stud_Num, Stud_Lname, Stud_Fname-

All the above three are super keys but

Stud_Num is the candidate key

Primary Key

A candidate key which is selected to uniquely

identify each row in the table


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It cannot contain null

Null

An unknown attribute value A known but missing attribute vale

A not applicable value

Secondary Key

An attribute or combination of attributes usedfor data retrieval purposes.

E.g. Although a Cust_Id is the primary key

users use Cust_Name and Cust_Lname to

retrieve data such keys are called secondary

keys.

Foreign Key

An attribute or combination of attributes in

one table whose values must match the

primary key in another table or be null

E.g Vend_Code in Product Table

Integrity Rules

Entity Integrity


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All primary key entries are unique and no part

of a primary key may be null

Use of Entity Integrity:

Each row will have a unique identity

Foreign key values can properly reference

primary key values

Referential Integrity

A foreign key may have either a null entry-as

long as it is not a tables primary key- or an

entry that matches the primary key value in a

table to which it is related.

Every non-null foreign key value must

reference an existing primary key value.

Other Integrity Rule

Other integrity rules that can be enforced in

the relational model are NOT NULL and

UNIQUE constraints.


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NOT NULL constraint can be placed on a

column to ensure that every row in the table

has a value for that column.

UNIQUE constraint is a restriction placed on a

column to ensure that no duplicate values exist

for that column.

Relational Set Operators

There are eight relational operators which

explain how the data is manipulated

theoretically.

SELECT, PROJECT, JOIN, INTERSECT,

UNION, DIFFERENCE, PRODUCT and

DIVIDE

1. UNION

Union combines all rows from two tables,

excluding duplicate rows.

The tables should be union-compatible

meaning:


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The two tables should have the same

number of columns; columns should have

same names and should have the samedomains.

Only then we can carry out union on the

two tables.

2. INTERSECT

It gives only the rows that appear in both

the tables. The tables have to be

compatible to use the intersect operation

on them.

Data types of the columns should also

match.

3. DIFFERENCE

It gives all the rows in one table that are

not found in the other table.

It subtracts one table from the other.


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Tables have to be union-compatible to

give valid results.

4. PRODUCT

It gives all possible pair of rows and

columns from the two tables. It is also

known as Cartesian product.

5. SELECT

It is also known as RESTRICT gives

values for all rows found in a table.

Or one can also select rows based on some

condition.(horizontal subset)

6. PROJECT

It gives all values for selected attributes.

It gives a vertical subset of a table.

7. DIVIDE


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It requires the use of one single column

table and another two column table.

Table with two columns is divided by table

with single column the resulting table will

contain those values where a value in the

unshared column is associated with every

value present in the table two.

Relationships in Relational Database

There are three types of relationships between

tables.

1: M -One to Many

1: 1- One to One

M: N- Many to Many


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The 1: M Relationship

Relationship between Painter and Painting is a1: M relationship.

One painter can paint many paintings and

Each painting can be painted by a one and

only one painter.

The One to Many relationships are

implemented by putting the primary key of the

1 side table in the table on the many side

as a foreign key as shown below:

Table Name: Painter

Primary key: Painter_Num

Painter_NumPainter_LnamePainter_Fname


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Table Name: PaintingPrimary key: Painting_Num

Foreign Key: Painter_Num

Painting_NumPainting_TitlePainter_Num

The 1: 1 Relationship

In this relationship one instance of an entity is

related to only one other entity instance.

E.g. one professor is a chairman of only one

Department and one department can only have

one chairman.

The M: N relationship

Consider the following example:


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A student can take many classes and a class

can have many students.

This is a many to many relationship between

STUDENT and CLASS table.

Logically the M: N relationship between the

STUDENT and CLASS table is as follows:

Table: STUDENT

Stud_NumStud_FnameStud_LnameClass_Code

Table:CLASS

Class_CodeStud_NumClass_TimeClass_Room


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The above table structures create a lot of

redundancies.

In the student table student data such as fname

lname is repeatedly stored for every class that

the student takes.

Similarly in the class table every student

taking a class generates a class record.

The above problems in the M: N relationship

can be avoided by creating a third entity called

as a composite entity orbridge entity.

This entity is implemented by creating a third

table called linking table.

Table: STUDENT

Primary key: Stud_Num

Stud_Num Stud_Fname Stud_Lname

Table: CLASS

Primary key: Class_Code


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Class_Code Class_Time Class_Room

Linking Table: Enroll

Primary key: Class_Code+Stud_Num

Foreign key: Class_Code, Stud_Num

Class_Code Stud_Num Grade

Enroll table must contain the primary key of

both tables which are linked.

Here in the new table they are the foreign

keys.

Also the combination of these primary keys is

the primary key of the new table.

The foreign key column may have repeated

values but those are controlled redundancies.

Controlled redundancy is required in a

database for increasing the performance

(processing speed) the database.


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Data DictionaryIt provides detailed information about all the

tables in the database.

It contains metadata-list of attributes and

characteristics of the attributes.

System Catalog contains data about all the

objects in the database.

Homonyms

Homonyms are the same names used to label

different attributes.

It should be generally avoided.


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Some relational software checks for

homonyms and alerts the user or makes the

corrections automatically.

Synonyms

The use of different names to identify the

same object is called synonym.

Synonyms

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