(1)Explain various types of Database Architecture.
Ans :-
Database Architecture
->The DBMS design depends upon its architecture. The basic client/server architecture is used to deal with a large number of PCs, web servers, database servers and other components that are connected with networks.
->DBMS architecture depends upon how users are connected to the database to get their request done.
->Database consists sensitive data which is accessed as per the user’s requirement.
->Three different type of Architecture available are :-
1. 1-tier
2. 2-tier
3. 3-tier

-> Database architecture can be seen as a single tier or multi-tier. But logically, database architecture is of two types like: 2-tier architecture and 3-tier architecture.

1-Tier Architecture :-
-> Simplest architecture of database.
->Client,Server and database all are available on single computer.
->User can access database directly.
->Changes which are implemented by user are directly affect the database.
-> Here the programmer directly accesses the database using sql queries.
-> The 1-Tier architecture is used for development of the local application, where programmers can directly communicate with the database for the quick response.

2. 2-tier architecture
-> Two different types of nodes are there – One is client and another is server.
-> Database system remeains on Server computer which is accessed
by user from client computer.
-> Application program and user interface remains on client computer.
->In the two-tier architecture, applications on the client end can directly communicate with the database at the server side. For this interaction, API’s like: ODBC, JDBC are used.
-> The server side is responsible to provide the functionalities like: query processing and transaction management.
-> User can directly connect with database system using application software.
->To communicate with the DBMS, client-side application establishes a connection with the server side.

3. 3-Tier :-
-> The 3-Tier architecture contains three different types of nodes are there – Client , Application Server and Database Server.
-> User cannot access database access database server directly. User has to send request to the Application server and application server finds data from database server and provide information to the client computer(to the user)
->More secure than other two architecture.
-> It is works as intermediator between Client and Database server.

(2)Explain various Keys with appropriate example.
Ans :-
Keys
-> Keys play an important role in the relational database.
-> It is used to uniquely identify any record or row of data from the table. It is also used to establish and identify relationships between tables.
(1)Primary Key (PK):
->A primary key uniquely identifies each record in a table.
->It must have a unique value for each record and cannot contain NULL values.
->It is the first key used to identify one and only one instance of an entity uniquely.
-> Example
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
Name VARCHAR(50),
Age INT
);

(2)Foreign Key (FK):

->A foreign key is a column or a set of columns in a table that refers to the primary key of another table.
->It establishes a relationship between two tables.
-> Example
CREATE TABLE Courses (
CourseID INT PRIMARY KEY,
CourseName VARCHAR(50)
);

CREATE TABLE Enrollments (
EnrollmentID INT PRIMARY KEY,
StudentID INT,
CourseID INT,
FOREIGN KEY (StudentID) REFERENCES Students(StudentID),
FOREIGN KEY (CourseID) REFERENCES Courses(CourseID)
);

3.Candidate Key:
->A candidate key is a column or a set of columns that can uniquely identify a record in a table.
->It is similar to a primary key but has not been selected as the primary key.
->A candidate key is an attribute or set of attributes that can uniquely identify a tuple.
-> Example
CREATE TABLE Employees (
EmployeeID INT,
SSN VARCHAR(15) UNIQUE,
Email VARCHAR(50) UNIQUE,
PRIMARY KEY (EmployeeID)
);

4.Super Key:
->A super key is a set of one or more columns that, taken together, uniquely identify a record in a table.
->It may contain more columns than necessary to uniquely identify a record.
->Super key is an attribute set that can uniquely identify a tuple. A super key is a superset of a candidate key.
->Example
CREATE TABLE Orders (
OrderID INT,
CustomerID INT,
ProductID INT,
OrderDate DATE,
PRIMARY KEY (OrderID),
UNIQUE (CustomerID, ProductID, OrderDate)
);
5.Composite Key:
->A composite key is a key that consists of multiple columns used to uniquely identify a record.
->Whenever a primary key consists of more than one attribute, it is known as a composite key. This key is also known as Concatenated Key.
->Example
CREATE TABLE Addresses (
Street VARCHAR(100),
City VARCHAR(50),
State VARCHAR(50),
PRIMARY KEY (Street, City, State)
);
6.Alternate Key:
->An alternate key is a candidate key that was not chosen as the primary key.
->It provides an alternative choice for the primary key.
->There may be one or more attributes or a combination of attributes that uniquely identify each tuple in a relation.
->One key is chosen as the primary key from these candidate keys, and the remaining candidate key, if it exists, is termed the alternate key.
->Example
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
RollNumber VARCHAR(10) UNIQUE,
Name VARCHAR(50)
);
7.Surrogate Key:
->A surrogate key is an artificial key (usually a numeric identifier) added to a table to serve as the primary key.
->It is often used when there is no suitable natural key.
->Example
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
FirstName VARCHAR(50),
LastName VARCHAR(50)
);
(3)Explain Mapping Cardinality with an example.
Ans:-
Mapping cardinality, also known as cardinality constraints, defines the relationship between entities in a data model by specifying the maximum and minimum number of instances of one entity that can be associated with a single instance of another entity.

Cardinality ratio is also called Cardinality Mapping, which represents the mapping of one entity set to another entity set in a relationship set. We generally take the example of a binary relationship set where two entities are mapped to each other.

There are four types of Cardinality Mapping in Database Management Systems:
1. One to one
2. Many to one
3. One to many
4. Many to many

-> One to One :-
->In a one-to-one relationship, each instance in the first entity is associated with at most one instance in the second entity, and vice versa.
->One to one cardinality is represented by a 1:1 symbol.
->For example :- one student can have only one student id, and one student id can belong to only one student. So, the relationship mapping between student and student id will be one to one cardinality mapping.
-> Example: Employee and Social Security Number (SSN)
Each employee is uniquely associated with one SSN.
Each SSN is uniquely associated with one employee.

-> One-to-Many (1:N) Mapping Cardinality:
->In a one-to-many relationship, each instance in the first entity can be associated with zero or more instances in the second entity, but each instance in the second entity is associated with at most one instance in the first entity.

-> One to one cardinality is the subset of One-to-many Cardinality. It can be represented by 1: N.
Example :-
(1)In a hospital, there can be various compounders, so the relationship between the hospital and compounders can be mapped through One-to-many Cardinality.
(2)Department (1) <—–> (*) Employees
Each department can have multiple employees.
Each employee is associated with only one department.

3. Many-to-Many (N:M) Mapping Cardinality:
->In a many-to-many relationship, each instance in the first entity can be associated with zero or more instances in the second entity, and vice versa.
OR
-> In many, many cardinalities mapping, there can be one or more than one entity that can associate with one or more than one entity of set 2. In the same way from the end of set 2, one or more than one entity can make a relation with one or more than one entity of set 1.

It is represented by M: N or N: M.

Students (*) <—–> (*) Courses
Each student can enroll in multiple courses.
Each course can have multiple students.

Many to One Cardinality:
In many to one cardinality mapping, from set 1, there can be multiple sets that can make relationships with a single entity of set 2. Or we can also describe it as from set 2, and one entity can make a relationship with more than one entity of set 1.

One to one Cardinality is the subset of Many to one Cardinality. It can be represented by M:1.

For example, there are multiple patients in a hospital who are served by a single doctor, so the relationship between patients and doctors can be represented by Many to one Cardinality.