# Database introduction
## What is a database?
* Is a collection of organized data that can be easily accessed and managed
* When it comes to databases we don't really interact with the database directly
* Instead we have a Database management system (DBMS)
* After which the DBMS will send the data over to the Database and return the results
* So... we always have a piece of software that interacts with the database
### Types of Databases
#### Relational Databases
* MySQL
* PostgreSQL
* Oracle
* SQL Server
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Note: Fundamentally all the Relational databases are the same at the core, however there are small differences on how SQL is implemented in each of the above examples.
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#### NoSQL
* MongoDB
* DynamoDB
* Oracle
* SQL Server
### Relational Databases & SQL
* Structured Query Language (SQL) is the language used to communicate with the DBMS
{% hint style="warning" %}
Each instance of any sort of Relational Database can be carved into multiple separate databases
* These databases are completely isolated from each other
* So we can have multiple apps with their own databases
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## Database Schema and Tables
### Tables
* A table represents a subject or event in an application
#### What does this mean?
Let's say we are building a E-Commerce application
We are going to have a table representing each part of the application
* Users - Table for all users that have registered
* Products - Tables for all the products that we sell
* Purchases - Table for all the purchases made
* All these tables will form a Relationship (Hence Relational Database)
If you think about it, a user will purchase a product and they will all be linked
* Every purchase order has to be associated with a user account
* A purchase order also has a list of products that the user wants to buy
{% hint style="warning" %}
It is very important that you think about these relationships before hand so you can design an efficient database
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### Columns vs Rows
* A table is made up of columns and rows
* Each Column represents a different attribute
* Each Row represents a different entry in the table
### DataTypes
* Databases have datatypes just like any other programming language
* In our case Postgres:
#### Why is this important?
* When you create a Column within a table, you need to specify what kind of DataType you want to use
### Primary Key
* Is a column or group of columns that uniquely identifies each row in a table
* Table can have one and only one primary key
* The Primary key does not have to be the ID column always. It's up to you to decide which column uniquely defines each record
* In the below example, since an email can only be registered once, the email column can also be used as the primary key
### Unique Constraints
* A UNIQUE constraint can be applied to any column to make sure every record has a unique value for that column
Primary Key has to be UNIQUE, but what happens when we have another row that needs to be unique?
* That's where the Unique Constraints come in
* In the above example we don't want duplicate names
* Once we apply the Unique Constraint, SQL will check to make sure that the new name we are trying to enter doesn't already exist in the database
### NULL Constraint
* By default when adding a new entry to a database, any column can be left blank
* When a column is left blank, it has a null value
* If you need column to be properly filled in to create a new record, a `NOT NULL` constraint can be added to the column to ensure that the column is never left blank
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