Compared to key-value and document databases, column-family stores impose more limitations on the structure of an aggregate. Specifically, column-family databases organize their columns into column families. Each column must be assigned to a particular column-family. Then, each column can be accessed via a column-family. In particular, accessing a column-family will return each of the columns associated with that column family.

At a high level point of view, a column-family database represents a map consisting of smaller maps, where the first map is a column family and the second map is a row. In the CQL(Cassandra Query Language) API, column families are referred to as tables. To gain some additional intuition about the data model, let's look at how data is stored in Cassandra:

-- create keyspace (database)
CREATE KEYSPACE hotel WITH replication =
    {'class': 'SimpleStrategy',
     'replication_factor': 3};

-- create table
CREATE TABLE hotel.employees (
    id text PRIMARY KEY,
    name text,
    roles set,
    salary smallint
)

-- insert row
INSERT INTO hotel.employees
       JSON '{"id": "10F-S53",
              "name": "Susan",
              "roles": {"accountant", "auditor"}}';

CQL is a typed language used for querying column-family databases in Cassandra. Meaning, Cassandra isn't really schemaless anymore, since data types and primary keys are required. In many ways, CQL feels like SQL, but deliberately excludes certain functionalities that violates their data model. In particular, CQL doesn't support group by operations, join operations, and others.

On the flip side, CQL supports certain functionalities that aren't supported in relational databases, since they violate the relational data model and comply with the column-family data model. For example, CQL supports the use of tuples and sets as data types defined within a schema.

Features of Column-Family Databases

• No transactions
• Writes are atomic at row level (cassandra) - any transformation of multiple columns of a single row is treated as a single write operation