Special relational operations | Relational Model

SPECIAL RELATIONAL OPERATIONS

These includes operations like
 

1. Selection
2. Projection
3. Join
4. Division

1. Selection-To identify a set of tuples which is a part of a relation and to extract only these tuples out. The select operation selects tuples that satisfy a given predicate or condition. It is also known as restriction operation. 

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Relational Model | Relational calculas

RELATIONAL CALCULUS

Relational algebra is concerned with a procedural language in which the user has to write the steps or procedure to obtain the required result.In this the user is not concerned with the detail of how to obtain information.

         In relational calculus the user tell his requirement and the output is available without knowing the method about its retrieval.

a.) It is a non procedural language.

b.) This concept was first expressed by codd. It is based on the predicate calculus.

c.) It is a formal language used to symbolize local argument in mathematics.Proposition specifying a property consist of an expression that names an individual object and an other expression called the predicate,that stands for the property that an individual object possesses.

In relational calculus we can make the following operations

Example-Let x and y are preposition then we can build other preposition like

"Not x"

"x and y"

 "x or y"m and so on.

Consider the following statement

India is a country.

We can write it.

" is a country  (India)"

we can drop is a then preposition will be

" Country (India)"

Finally if we use symbols for both predicates and objects then we can rewrite the statement X (a).Lower case letter denotes the variables beginning letters {a,b,c......}denotes the constants.

Upper case letter denotes the predicate.

P{X} where X is the argument.

It can be categorized into 2 parts:-

1,) Tuple Oriented Relational calculus

2.) Domain Oriented Relational calculus.

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Relational Model | Integrity constraints over relation

INTEGRITY CONSTRAINTS OVER RELATION

INTRODUCTION

Database integrity refers to the validity and consistency of stored data. Integrity is usually expressed in terms of constraints, which are consistency rules that the database is not permitted to violate. Constraints may apply to each attribute or they may apply to relationships between tables.

Integrity constraints ensure that changes (update deletion, insertion) made to the database by authorized users do not result in a loss of data consistency. Thus, integrity constraints guard against accidental damage to the database.

EXAMPLE- A brood group must be ‘A’ or ‘B’ or ‘AB’ or ‘O’ only (can not any other values else).

TYPES OF INTEGRITY CONSTRAINTS

Various types of integrity constraints are-

1. Domain Integrity
2. Entity Integrity Constraint
3. Referential Integrity Constraint
4. Key Constraints

1. Domain Integrity- Domain integrity means the definition of a valid set of values for an attribute. You define data type, length or size, is null value allowed , is the value unique or not for an attribute ,the default value, the range (values in between) and/or specific values for the attribute. 

2. Entity Integrity Constraint- This rule states that in any database relation value of attribute of a primary key can't be null.

EXAMPLE- Consider a relation "STUDENT" Where "Stu_id" is a primary key and it must not contain any null value whereas other attributes may contain null value e.g  "Branch" in the following relation contains one null value.

Stu_id	Name	Branch
11255234	Aman	CSE
11255369	Kapil	ECE
11255324	Ajay
11255237	Raman	CSE
11255678	Aastha	ECE

3.Referential Integrity Constraint-It states that if a foreign key exists in a relation then either the foreign key value must match a primary key value of some tuple in its home relation or the foreign key value must be null.

The rules are:

1. You can't delete a record from a primary table if matching records exist in a related table.

2. You can't change a primary key value in the primary table if that record has related records.

3. You can't enter a value in the foreign key field of the related table that doesn't exist in the primary key of the primary table.

4. However, you can enter a Null value in the foreign key, specifying that the records are unrelated.

EXAMPLE-Consider 2 relations "stu" and "stu_1" Where "Stu_id " is the primary key in the "stu" relation and foreign key in the "stu_1" relation.

Relation "stu"

Stu_id	Name	Branch
11255234	Aman	CSE
11255369	Kapil	ECE
11255324	Ajay	ME
11255237	Raman	CSE
11255678	Aastha	ECE

Relation "stu_1"

Stu_id	Cource	Duration
11255234	B.TECH	4 years
11255369	B.TECH	4 years
11255324	B.TECH	4 years
11255237	B.TECH	4 years
11255678	B.TECH	4 years

Examples

Rule 1. You can't delete any of the rows in the ”stu­” relation  that are visible  since all the ”stu” are in use in the “stu_1” relation.

Rule 2. You can't change any of the ”Stu_id” in the “stu” relation since all the “Stu_id” are in use in the ”stu_1” relation.

Rule 3. The values that you can enter in the” Stu_id” field in the “stu_1” relation must be in the” Stu_id” field in the “stu” relation.

Rule 4 You can enter a null value in the "stu_1" relation if the records are unrelated.

4.Key Constraints- A Key Constraint is a statement that a certain minimal subset of the fields of a relation is a unique identifier for a tuple.

There are 4 types of  key constraints-

1. Candidate key.
2. Super key
3. Primary key
4. Foreign key

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Relational Model | ER to Relational model conversion

ER TO RELATIONAL MODEL CONVERSION

INTRODUCTION- In the E-R model, data is represented using entities, and relationships are defined between these entities. However, with the relational model, the entities and their relationships follow strict guidelines. Usually, an E-R model is first developed, and then it is transformed into a relational model.

RULES TO CONVERT  ER TO RELATIONAL MODEL CONVERSION-

•        Build a table for each entity set
•         Build a table for each relationship set if necessary (more on this later)

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Relational Model | Key constraints | Types of key

KEY CONSTRAINTS

INTRODUCTION-A Key Constraint is a statement that a certain minimal subset of the fields of a relation is a unique identifier for a tuple.

TYPES OF KEY CONSTRAINTS- Various types of key constraints are-

1. Candidate key
2. Super key
3. Primary key
4. Foreign key

1. Candidate key- These are those attributes of a relation that contains the properties of uniqueness and irr-reducability. 

• Uniqueness- At any time no two tuples of a relation should have the same value means no duplicate values. 
• Irr-reducability-  Let "k" be the set of attributes in any relation "R" i.e.it contains composite key  then no proper subset of "k"  has the uniqueness property.

2. Super Key-  These are those attributes of a relation that contains the properties of uniqueness but not necessary that it satisfies  irr-reducability property.

3. Primary Key- Mixture of uniqueness and not null is known as the primary key.

Properties of Primary key-

1. Stable- It should be stable means value must not change or it should not be null throughout the life of an entity.

2. Minimal- It should be minimal means it should be composite of minimum no of fields that ensures that the occurrence are unique.

3. Definitive- It should be definitive means a value must exist for every record at the creation time.

4. Accessible-It should be accessible means any one who wants to create,read or delete a record mus be able to see the primary key values.

SYNTAX- create table <table name> (column name1 datatype (size), column name2 datatype(size) constraint (constraint name)  primary key);

EG- create  table  mydoc3 ( username varchar (20) ,password  number(10) constraint  rollno_pk  primary key);

4. Foreign key-Primary key of one table is the foreign key of some other table.

Features of foreign key:

1. Records cannot be inserted into a detail table if corresponding records in the master table do not exist.

2 .Records of the master table cannot be deleted  or updated if corresponding records in the detail table actually exist.

                        SYNTAX- create table <table name> (column name1 datatype (size), column name2 datatype(size) constraint (constraint name) primary key);

create table <table name> (column name1 datatype (size), column name2 datatype(size) references <tablename which contain primary key < primary key attribute>);

 EG- create  table  doc9    (bname  varchar(20),brollno number(20)    constraint vn_o   primary key);

create  table  doc10   (sname  varchar(20),srollno number(20)    references  doc9(brollno)); 

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