When would you use a relational database such as MySQL or Oracle versus some other NoSQL database?
Generally speaking, I would use a relational database in situations where data size and types are somewhat fixed and the user is entering data in a form-like interface. On the other hand, I would use NoSQL databases in scenarios where the size of the database needs to scale up, such as a forum or social media site.
How would you decide whether to use a document database like MongoDB versus a wide column database like Cassandra?
I would use MongoDB in scenarios where many different types of data needed to be stored, and I would use Cassandra where multiple, large instances of similar data needed to be stored.