It ranges from beginners to Advanced
For this project I have used postgresql.
Sites I have referred for this project links are provided below:
Employee table :https://www.w3schools.com/SQL/TRYSQL.ASP?FILENAME=TRYSQL_SELECT_ALL
Drinks Table : https://pgexercises.com/
General lessons on SQL:https://sqlzoo.net/wiki/SELECT_within_SELECT_Tutorial