MongoDB vs MySQL

The article entitled: What the heck are you actually using NoSQL for? does a very good job at presenting the pros and cons of using NoSQL.

Edit: Also read http://blog.fatalmind.com/2011/05/13/choosing-nosql-for-the-right-reason/ blog post too

Re-edit: I found some recent material (published in 2014) on this topic that I consider to be relevant: What’s left of NoSQL?

I don't know much of the underlying theory. But this is the advice I got: only use MongoDB if you run it across multiple servers; that's when it'll shine. As far as I understand, the NoSQL movement appeared in no small part due to the pain of load-balancing relational databases across multiple servers. So if you're hosting your application on no more than one server, MySQL would be the preferred choice.

The good people over at the Doctrine project recently wrote a quite useful blog post on the subject.

From what I have read so far... here is my take on it.

Standard SQL trades lower performance for feature richness... i.e. it allows you to do Joins and Transactions across data sets (tables/collections if you will) among other things.

This allows a application developer to push some of the application complexity into the database layer. This has it's advantages of not having to worry about data integrity and the rest of the ACID properties by the application by depending upon proven technology.The lack of extreme scalability works for pretty much all projects as long as one can manage to keep the application working within expected time limits, which may sometimes result in having to purchase high performance/expensive relational database systems.

On the other hand, Mongo DB, deliberately excludes much of the inherent complexity associated with relational databases, there by allowing for better scalable performance.

This approach forces the application developer to re-architect the application to work around the lack of relational features... which in and itself is a good thing, but the effort involved is generally only worth it if you have the scalability requirements. Please note that with MongoDB depending upon the data requirements w.r.t ACID properties, the application will have to step up and handle as necessary.