I have an opinion: most people learned about relational databases as if RDBMSes were designed to store the ultimate truth about some data. They figured that once the schema had been properly diagrammed and normalized, then they could load all their data into it, and finally, start doing some queries.
To pick on an easy target, look at Wikipedia's article on schema design. It summarizes the two steps a designer must take:
- Determine the relationships between the different data elements.
- Superimpose a logical structure upon the data on the basis of these relationships.
Do you see a step that's missing? If you've deployed and maintained a large-scale application you'll probably see what the Wikipedia authors omitted. In fact, it's the first step: Figure out what one question your database must answer. Then, design your schema to answer that question as fast as possible. And now you're done. Come to think of it, you never had to do steps 1 and 2 at all.
There's a total disconnect between the approaches of introductory SQL courses and real-world application development, and I think this disconnect is slowing down adoption of NoSQL.
Consider Facebook Messages. After a (now rather well-publicized) evaluation process, Facebook chose HBase, a NoSQL data store, as the main database for their message system. I haven't talked to anyone there, but I figure they chose it based on this criterion:
How fast will our database answer the question, "What are this user's most recent 10 messages?"
They chose the database system that could answer that question the fastest, and they designed the best schema they could think of to answer that question. Anything else they need to ask HBase may be slow, or difficult, but that doesn't matter, because "What are this user's most recent 10 messages?" probably accounts for 99% of the load on their system.
If you learned about databases in college, following some textbook, I expect you were guided through a long process of modeling real-world data using rows and columns, to express some profound truth about the data. Then, you were introduced to SQL, with which you could query the data. At the end of the course, maybe there was a brief discussion of database performance. Probably not.
Data at the scale that the largest websites handle doesn't work that way. Large applications design their schemas to answer one question as quickly as possible, and no other considerations are significant.
The next time you read about a NoSQL database you might wonder, "What about foreign keys, or normalization? What about transactions? Why can't I define secondary indexes? Why are range queries prohibited?" (I'm just picking some limitations at random—each system is different.) Consider who built these new database systems, and what their experience has been. The ideas behind NoSQL databases mostly originated at places like Google, Amazon, and Yahoo. They build huge systems, and huge systems' loads are usually dominated by a handful of queries. Companies build their database systems from the ground up to optimize the performance of these queries. NoSQL databases encourage you to figure out ahead of time, "What one question do I need to answer?" Figure that out, and choose your database software and your schema based on that. Nothing else really matters.