Deciding between a NoSQL database or a relational database system is about understanding the trade-offs that led to the creation of NoSQL to begin with. NoSQL systems have advantages over traditional SQL databases because they give up certain RDBMS features in order to gain other performance, scalability and developer usability capabilities.
What NoSQL gives up (this varies by NoSQL engine) :
- Relationships between entities (like tables) are limited to non-existent. For example, you usually can't join tables or models together in a query. Traditional concepts like data normalization don't really apply. But you still must do proper modeling based on the capabilities of the particular NoSQL system. NoSQL data modeling varies by product and whether you are using a document vs column based NoSQL engine. For example, how you might model your data in MongoDB vs HBase varies because each solution offers significantly different capabilities.
- Limited ACID transactions. The level of read consistency and atomic write/commit capabilities across one or more tables/entities varies by NoSQL engine.
- No standard domain language like SQL for expressing ad-hoc queries. Each NoSQL has its own API and some of the NoSQL vendors have limited ad-hoc query capability.
- Less structured and rigid data model. NoSQL typically forces/gives more responsibility at the application layer for the developer to "do the right thing" when it comes to data relationships and consistency. Think of NoSQL as a schema on read instead of the traditional schema on write.