In the previous chapter, you learned the basic concepts of object-oriented programming. These include class and methods, and how they are connected by generic functions in R through method dispatch. You learned about the basic usage of S3, S4, RC, and R6, including defining classes and generic functions as well as implementing methods for certain classes.
Now that we have covered most of the important features of R, it is time we go ahead and discuss more practical topics. In this chapter, we will begin the discussion with how R can be used to work with databases, which is perhaps the first step of many data-analysis projects: extracting data from a database. More specifically, we will cover the following topics:
Understanding relational databases
Using SQL to query relational databases such as SQLite and MySQL
Working with NoSQL databases such as MongoDB and Redis
