Our application's database is much more than a simple record of things that we need to save for future retrieval. If all we needed to do was save and retrieve data, we could easily use flat text files. The fact is, though, that we want to be able to perform interesting queries on our data. What's more, we want to do this efficiently and without reinventing the wheel. While non-relational databases (sometimes known as NoSQL databases) are very popular and have their place in the world of the web, relational databases long ago solved the common problems of filtering, sorting, aggregating, and joining tabular data. Relational databases allow us to define sets of data in a structured way that maintains the consistency of our data. Using relational databases also gives us, the developers, the freedom to focus on the parts of our app that matter.

In addition to efficiently performing ad hoc queries, a relational database server will also do the following: