Relational joins

In real-world applications, different pieces of data relate to each other in a variety of ways. Relational databases allow us to perform queries that make these relationships explicit; for instance, to retrieve a set of events whose location is in the state of New York (this is assuming events and locations are different record types). Cassandra, however, is not a relational database and does not support anything such as joins. Instead, applications using Cassandra typically denormalize data and make clever use of clustering in order to perform the sorts of data access that would use a join in a relational database.

For data sets that aren't already denormalized, applications can also perform client-side joins, which mimic the behavior of a relational database by performing multiple queries and joining the results at the application level. Client-side joins are less efficient than reading data that has been denormalized in advance, but they offer more flexibility. We'll cover both of these approaches in Chapter 6, Denormalizing Data for Maximum Performance.