What is a database ?
A database is an organized collection of information for  easy access, management, and update.

Types of Database Systems

• Flat file system
• Hierarchical Model
• Network Model
• Relational Model
• Object Relational Model
• No SQL Data model

Flat file database

• The data stored in a ordinary "flat file".  (names.txt)
• Example: contact list.
• The whole file need to be read to read and process data.
• Takes longer time to read large sets of data.
• Data limited by memory size.
• Good for small amount of data

Hierarchical Database

Similer to the folder structure on a computer.
Every folder can contain a sub-folder and each sub-folder can still hold more sub-folders. Finally, in some folders, we will store files. Every child node (sub-folder) will have a single parent (folder or sub-folder). This creates a hierarchy of the dataset.

Object-oriented Database

• The Object and its data accessed through pointers.
• Support diverse structures and extendable.
• The data and the program operate as one.
• This model treat the data as native code.

Relational Database Systems

Most widely used . Easy to use.
Data stored in rows and columns in tables.
The data have relationships with other tables (Class > Students > Subjects > Marks)
Data stored is fixed predefined structures . Many empty spaces
Data manipulated using Structured Query Language (SQL).

Examples:
Dbase III+, MS Access, Oracle, Ms SQL Server, mySQL , IBM DB2, SQLite, and PostgreSQL



No SQL Database
Keeps data in key = value pairs.
Highly flexible structure
Scalable
Facebook stores terabytes of additional data every day using no SQL



Ex. Mongo DB