Cassandra: The Definitive Guide, 2nd Edition

Chapter 4. The Cassandra Query Language

In this chapter, you’ll gain an understanding of Cassandra’s data model and how that data model is implemented by the Cassandra Query Language (CQL). We’ll show how CQL supports Cassandra’s design goals and look at some general behavior characteristics.

For developers and administrators coming from the relational world, the Cassandra data model can be difficult to understand initially. Some terms, such as “keyspace,” are completely new, and some, such as “column,” exist in both worlds but have slightly different meanings. The syntax of CQL is similar in many ways to SQL, but with some important differences. For those familiar with NoSQL technologies such as Dynamo or Bigtable, it can also be confusing, because although Cassandra may be based on those technologies, its own data model is significantly different.

So in this chapter, we start from relational database terminology and introduce Cassandra’s view of the world. Along the way we’ll get more familiar with CQL and learn how it implements this data model.

The Relational Data Model

In a relational database, we have the database itself, which is the outermost container that might correspond to a single application. The database contains tables. Tables have names and contain one or more columns, which also have names. When we add data to a table, we specify a value for every column defined; if we don’t have a value for a particular column, we use null. This new entry adds a row to the ...