As someone who has a masters in Computer Engineering and well familiar with pros and cons of entity relationship databases. I quickly drew out some tables on the whiteboard for users, projects and so on with primary keys and foreign keys. In the last few years, I've spent more and more time in no sql databases, document oriented databases and column oriented databases, and I quickly took the opportunity to impress the interviewers how we can represent this in a non traditional databases, by eschewing ER. Big mistake on my part by trying to impress the interviewers, according to HR, I've struggled with relational databases, which is ironic because I thought this was the easiest question during the whole interview. lol. Again, one of the interviewers couldn't or wouldn't accept the cons of ER databases, especially how a designer has to know full structure of the schema in the beginning of a project and making schema changes later on can have big effect on your code. I should have just concentrated on my ER diagram more instead of going in document oriented databases or column store databases vs row-store databases.