In the What Cassandra offers, and what it doesn't section of Chapter 1, Getting Up and Running with Cassandra, you learned that Cassandra is not a relational database, despite some surface similarities. Specifically, this means that Cassandra does not have a built-in concept of the relationships between data in different tables. There are no foreign key constraints and there's no JOIN clause available in the SELECT statements; in fact, there is no way to read from multiple tables in the same query. Whereas relational databases are designed to explicitly account for the relationships between data in different tables, whether they're one-to-one, one-to-many, or many-to-many. Cassandra has no built-in mechanism for describing or traversing inter-table relationships.

That being said, Cassandra's compound primary key structure provides an ample affordance for a particular kind of relationship—the parent-child relationship. This is a specific type...