# 1. Sql - Introduction 

**What is Data and What is Strcutured Data ?**  
Data - piece of information that can be stored and processed.  
Strcutured data - data that is organized in a predefined format so that it can be easily stored and processed.  
Ex: DB table is the example of structured data

**WAYS TO KEEP DATA STRCUTURED ?**

1.  Database Tables (Most Common) - Keep data in tables in relational databses like MySql and postgres
    
2.  JSON Format
    

**WHY SQL NEEDED ? Why no just excel files ?**  
1\. Stored Structured Data  
2\. Query the Data  
3\. Update/Delete Data efficiently  
4\. Provide security as only that person access data who have acces of it.  
\-> In Sql make searching fast while in excel/file have slow.  
\-> Sql have millions of rows while excel dont have if we even try then it is difficult it maintain such huge data in excel  
\-> Sql have high security but excel have just basic security.

One Line -> Sql handle large complex datasets while excel handle only small simple datasets.

**DATABASES**  
Organised collection of data.

**RELATIONAL(SQL) vs NON RELATIONAL(NO SQL)**  
SQl -> Data stored in tables , Data is highly strcutured , Uses in banking and e-commerce transactions  
NoSQL -> Data stored in documents , key-value pairs , Data is less sstructred , Uses in Real time application , socila media etc. , (More flexible)

**WHAT IS SQL ?**  
SQL is a structured query language used to intract with relational databases.  
It makes data retrival efficient , updateion , deletion efficient.

**Relational DataBase Management System (RDMS)**  
It is software used to store, manage, and retrieve data in the form of **tables**, where tables can be related to each other using keys.  
Ex: MySQL -> Popular open source RDMS used in WEB DEV  
PostgresSQL -> Advance open source RDMS , used to handle complex queries  
SQLIte -> Used in mobile /local setups  
Oracle -> enterprise level , SQLServer

**SCHEMA**  
A schema is like the blueprint of your database.  
It describe tables , fields , relationship  
Think of it like a map that tell the database how your data is orgainised.

**DATABASE NORMALIZATION**  
Database Normalization is the process of organizing data in a database to reduce redundancy (duplicate data) and improve data integrity.

Need of Normalization real world example:  
For an e-commrece platform you want to store data of customer  
If we store data in a table like this : customerid | customerName | Mobile | Product  
then there is a chances of redundancy(duplicate customer name)  
Intead of this we can store data in multiple tables(normalize this) like this:  
Customers -> cusomerId | Name | Phone  
Product -> Id | Name | Rating  
Order -> OrderId | CustommerId  
This will avoid redundancy

Anomaly -> Inconsistencty

**FORMS/RULES of NORMALIZATION**  
We can achieve normalization from following ways -:  
1\. 1NF (First Normal Form):  
Each col should contain atomic(individual) values.

![](https://cdn.hashnode.com/uploads/covers/65c53497c49d93a3d3eb4afe/aaef9d1b-507a-4e2e-b2fd-7e3ffc505362.png align="center")

Still above have some problems

2\. 2NF (Second Normal Form)  
Must already in 1NF + No Partial dependecy -> which means A non-key column should depend on the **whole primary key(composite key)**, not part of it.

![](https://cdn.hashnode.com/uploads/covers/65c53497c49d93a3d3eb4afe/55f70158-f203-4521-af41-0a349dc8a549.png align="center")

3.  3NF  
    Already in 2NF + No Transitive dependecy which means Every non key attribute depends only on the primary key.
    
    ![](https://cdn.hashnode.com/uploads/covers/65c53497c49d93a3d3eb4afe/936c3e4a-ff2a-40b8-a8d2-9805b71c614c.png align="center")
    
4.  BCNF (Boyece codd normal form)  
    Advance version of 3NF : every determinant should be a candidate key.
    
    Example of Normalization for All : TODO
    

**Entity Relationship(ER) Diagrams**  
It is the visual representation of how entities (tables) related to each other.  
Entity -> Table  
Attributes -> Columns  
Relationships-> How tables connect  
  
**PRIMARY KEY**  
A column (or set of columns) that uniquely identifies each row in a table.  
\-> Can not be null  
\-> Must be unique  
  
**FOREIGN KEY**  
A column in one table that refers to the **Primary Key** of another table.  
It creates a relationship between tables and maintains **referential integrity**.  
  
**RELATIONSHIP TYPES**  
1\. One to One (1:1) -> One record in Table A is related to only one record in Table B, and vice versa.  
Ex: We have user and Passport table ; In user table we have passport id as foreign key that mean one use have only one passport which is one to one.  
  
2\. One to Many(1:M) -> One record in Table A can be related to many records in Table B.  
Ex: One teacher can teach many students ; We have teacher table and student table ; In student table we have same teacher ids with different students that mean one teacher related to many student:  
  
3\. Many to Many (M:M) -> Many records in Table A can be related to many records in Table B.  
Ex: A student can have many courses and a course can have many students.

![]( align="center")

(Table meh 2ono col me same chez repeat ho jaay)  
  
TODO: ER diagrams practice  
TODO: Convert ER Diagrams to SQL Table  
TODO: Normalization