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https://github.com/makstyle119/database-design


https://github.com/makstyle119/database-design

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README 

        
# Database Design



## Question & Answer



**Q1- What Is Data ?** 


**A1- Data** is anything. neither it have value or not.

**Example:**

- name of your all classmates

- name of your favorite cartoon character

- your password

- your random thoughts 

- In simple anything can be data.



**Q2- What Is Database ?** 


**A2- Database** is something that store data, In computer science it commonly refer as the a server where we save our product (website/app/dashboard) data.



**Q3- What is an Entity ?** 


**A3- Entity** is anything we store data about.

**Example:**

- person

- car



**Q4- What is an Attribute ?** 


**A4- Attribute** are the things we store.

**Example:**

- name

- email

- color



**Q5- What is a Row ?** 


**A5-** A **Row** is the value (group of values) against a entity inside a table.

**Example:**

- person's name

- person's email



**Q6- What is a Column ?** 


**A6-** A **Column** is the value (group of values) against a attribute inside a table.

**Example:**

- name of all persons



**Q7- What is a Table ?** 


**A7-** A combination of rows and column to save entities.



**Q7- What is a View ?** 


**A7- View** is a virtual representation of a table with a selected columns.



**Q8- What is SQL - (Structured Query Language) ?** 


**A8- SQL - (Structured Query Language)** is a Language to communicate with SQL database:

- use to define database structure (DDL)

    - to create the table structure

- use to manipulate the data inside the database (DML)

    - to perform crud on the database

    - connection between the database



**Q9- What is Data Integrity ?** 


**A9- Data Integrity** is all data is correct, up to date.

- It has 3 types:

    1. Entity Integrity

        - in simple unique entity

            - every entity should have a unique identifier (mostly id/uuid)

    2. Referential Integrity

        - relations/foreign keys are referential integrity

    3. Domain Integrity

        - data type is domain integrity (int(11)/varchar(255))

            - few of data types are:

                - int = integer

                - char = string

                - null = no value

                    - nullable means this column is optional 

- Errors in database called anomaly

    - Anomaly = when database behave in unexpected way



**Q10- What is a good Database Design ?** 


**A10-** A **Database Design** is something which have data integrity, and no repeated data, no stale data.

In general database design break into 3 sections (schemas):

1. Conceptual

    - relation falls here

2. logical

    - table structure falls here

3. Physical

    - what database/data types/server/other stuffs falls here

--- 

We can understand them as conceptual schema is general thinking to physical is specific thinking.



**Q11- What is Relationship in a Relational Database ?** 


**A11-** In a **Relational Database** we have multiple types of Relationship.

1. One-To-One

    - formally written as `1:1`

    - is one entity have one attribute

        - one username

    - if anything have one-to-one relation we will add a new column in the same table

    - but sometime we save one-to-one relation in different tables

        - one person can have one driving license number

            - person will save in user table

            - driving license number will save in driving license table

    - usually between one or two tables (binary relation)

2. One-To-Many

    - formally written as `1:N`

    - here one entity have many attributes

        - one person can have multiple comments in a blog

        - one person can have multiple orders

    - if anything have one-to-many relation we will add a foreign key in the table

    - one-to-many also know as parent-child relation

        - parent is one entity and have primary key

        - child can be multiple entities and have foreign key

    - child have also refer to parent but not vice versa

    - between two tables (binary relation)

3. Many-To-Many

    - formally written as `M:N`

    - here many entity have many attributes

        - one class can have multiple students

        - but one student can have multiple classes as well

    - if anything have many-to-many relation we will add a bridge/intermediate table.

        - this table will have student_id and class_id as well.

    - usually between three or more tables but every two tables will have one-to-many relationship between primary and bridge table (binary relation)



**Q12- What is Index ?** 


**A12- Index** are same as in books, to find something quickly, it will point you to the data.

- **Unique Index**

    - this index will make sure that all values are unique



**Q13- What is Primary Key ?** 


**A13- Primary Key** is the unique identifier of the entity use to make all foreign relations.



**Q14- What is Look-Up Table ?** 


**A14- Look-Up Table** is a simple table consistence of **List Of Values**. 


- Relation

    - Look-Up tables work with One-To-Many relationship

- Example:

    - Status

    - Gender



**Q15- What are Keys ?** 


**A15- Keys** are unique, Never Change and Never Null. (Recommended to index most of your keys)

- **characteristics**

    - **Not Null** is a column that cannot have null value

    - **Unique** is a column that cannot have duplicate value

- **SuperKey** is any number of columns that forces every row to be unique

    - **Requirement**

        - Can every row be unique

        - How any column are needed

    - **Note:** SuperKey are only for designing the database and not practical

- **Candidate Key** is minimum number of columns that forces every row to be unique

- **Primary Key** is the candidate key which we will use to make foreign relations

    - mostly a id column

- **Alternate Key** is all other candidate keys except the primary key

- **Natural Key** is a candidate key which is already present in the database and a real world value.

    - username

    - email

- **Surrogate Key** is a key that is not present in the real world.

    - mostly a id column

- **Foreign Key** is primary key use in another table to make foreign relations, nullable, can be repeated (because of binary relation) and changeable.

- **Simple Key** is refer as single attribute with in a table to make each row unique.

- **Composite Key** is refer as a combination of more than one attribute with in a table to make each row unique.

- **Compound Key** is a key with multiple column and every column is a key.



## key Points

- *One Entity - (table) should only contain relevant data*



## fun facts

- *Relational Database come from math relation not the relation between tables*



## Key words

- *tuple = use for row*

- *DBMS = Database Management System*

- *RDBMS = Relational Database Management System*

- *DDL = data definition language*

- *DML = data manipulation language*

- *Normalization = step to follow in database design*

- *key = unique identifier*



## Style Guide

- *save everything in atomic level*

### SQL

- `--` this is a single line comment = use for single line comments

- `/*

    this is a multi-line

    comment

  */` = use for multi line comments

- normally we use uppercase for build-in keywords, eg: `SELECT`, `UPDATE`, `JOIN`

- normally we use lowercase for keyword for things we name: `user`, `role`

- normally we use `kabab case` in naming conventions in sql, eg: `user_role`