Joins

Database indexing

Database indexing for dumb developers
How do SQL indexes work
Socratica
How query on a column with non-clustered index works

Indexing and comparing perfomances before and after indexing

Code
Results below
Another example of how indexing works and speeds up queries

ER diagrams

GeeksforGeeks Introduction of ER Model

YouTube Giraffe Academy
Creating ER diagram of the company database
Creating relations(tables) from the diagram
Company database with inserted rows

More coding

1 hour of SQL