Implement a Repository containing relevant SQL Queries

[Sarvesh67]

Is your feature request related to a problem? Please describe.
The chain of SQL queries necessary to generate the final ER is missing currently.

Describe the solution you'd like
Include all the common as well as the database specific SQL queries into the repository. Finally all the queries will be executed using the queryPromise method in the database services and be returned to the respective controllers.
See to it that the paradigm being used to implement repositories decouples itself from the controllers as well as the database services allowing us to easily update and add SQL queries.

Additional context
Below is a roadmap developed containing the current SQL queries. Note dow the additional ones by starting a discussion here.