In ER Model, relationship exits between two entities.
Types :
(i) one-to-one Relationship,
(ii) one-to-many Relationship,
(iii) many-to-many Relationship.

ACID Properties :
(i) The acronym stands for Atomicity, Consistency, Isolation and Durability.
(ii) Atomicity follows the thumb rule "All (or) Nothing" while updating the data in database for the user performing the update operation.
(iii) Consistency ensures that the changes in data value to be constant at any given instance. Isolation property is needed during concurrent transaction.
(iv) Durability is defined as the system is ability to recover all committed transactions during the failure of storage or the system

Data models define how the logical structure a database is modeled. Data models define how data is connected to each other and how they are  processed andstored inside the system
Types of Data Model :
1) Hierarchical database model 
2) Network model 
3) Relational model 
4) object oriented database model

(i) USE Database: This command is used to select the database in MySQL for working.
Syntax:
mysql>use test:
Database changed
mysql>
(ii) SHOW Database: Lists all the databases available in the database server.
Syntax:
mysql>show databases;
(iii) SHOW Tables: Lists all the tables available in the current database we are working in.
Syntax: mysql>show tables;

Cardinality is defined as the number of items that must be included in a relationship (i.e) number of entities in one set mapped with the number of entities of another set via the relationship The three classifications in Cardinality are one-to-one, one-to-many and Many-to-Many.
In the above example we have two entities Person and Vehicle. If we consider a person driving vehicle, then we have one-to-one relationship between Person and Vehicle.
In the above example, Customer places the Order is a one-to-many relationship. Here the customer can place multiple orders and the order is related to only one customer.
The example of many-to-many relationship is Students registering the Courses. A student can register more than one courses and A course can be registered by many students. Hence it is many-to-many.

Evolution of DBMS:
(i) The concept of storing the data started before 40 years in various formats.
(ii) In earlier days they have used punched card technology to store the data. Then files were used.
(ii) The file systems were known as predecessor of database system.
(iv) Various access methods in file system were indexed, random and sequential access.
The file system had more limitations like.
Data Duplication : Same data is used by multiple resources for processing, thus created multiple copies of same data wasting the spaces.
High Maintenance : Access control and verifying data consistency needs high maintenance cost.
Security : Less security provided to the data.
(v) So database systems became popular to overcome the above limitations of file system.

An attribute is the information about entity and it will describe, qualify, quantify, classify, and specify an entity.
An attribute will always have a single value, that value can be a number (or) character (or) string.
Types of Attributes: 
(1) Key Attribute
(2) Simple Attributes
(3) Composite Attributes
(4) Single Valued Attribute
(5) Multi Valued Attribute.
1 Key Attribute:- Generally a key attribute describes a unique characteristic of an entity.
2. Simple Attributes:- The Simple Attributes cannot be separated it will be having a single value for their entity.
For Example: Let us consider the name as the attribute for the entity employee and here the value for that attribute is a single value.

3. Composite Attributes:- The composite attributes can be subdivided into simple attributes without change in the meaning of that attribute.
For Example: In the above diagram the employee is the entity with the composite attribute Name
which are sub-divided into two simple attributes first and last name.
4. Single Valued Attributes:- A single valued attribute contains only one value for the attribute and they don't have multiple numbers of values.
For Example: Age- It is a single value for a person as we cannot give n number of ages for a single

5. A Multi Valued Attributes:- A multi valued attribute has more than one-value for that particular attribute.
For Example: Degree A person can hold n number of degrees so it is a multi-valued attribute.

The database technology came into existence in terms of models with relational and object- relational behavior. The major database models are listed below:
1. Hierarchical Database Model:
(i). The famous Hierarchical database model was IMS (Information Management System), IBM's first DBMS.
(ii). In this model each record has information in parent/child relationship like a tree structure.
(iii). The collection of records was called as record types. This individual records are equal to rows.

Advantages: Less redundant data, efficient search, data-integrity and security.
Limitations: Complex to implement and difficulty in handling many to many relationship.
2. Network model:
(i). The first developed network data model was IDS (Integrated Data Store) at Honeywell.
(ii). Network model is similar to Hierarchical model except that in this model each member can have more than one owner.
(iii). The Many to Many relationships are handled in a better way.

(iv). This model identified by the three database components Network schema, Sub schema and Language for data management.
Network schema: schema defines all about the structure of the data base.
Sub schema: controls on views of the database for the user.
Language: basic procedural for accessing the database.
3. Relational model:
Oracle and DB2 are few commercial relational models in use Relational model is defined with two terminologies: Instance and Schema.
Instance: A table consisting of rows and columns.
Schema: Specifies the structure including name and type of each column.

4. Object oriented database model:
(i). This model incorporates the combination of Object Oriented Programming (OOP's) concepts and database technologies.
(ii). Practically, this model serves as the base of Relational model. Object oriented model uses small, reusable software known as Objects.
(iii). These are stored in object oriented database.