How Can I Search for Records That Have A Null/Empty Field Using CQL?

Question

How can I write a query to find all records in a table that have a null/empty field? I tried tried the query below, but it doesn't return anything.

SELECT * FROM book WHERE author = 'null';

Answer 1

null fields don't exist in Cassandra unless you add them yourself.

You might be thinking of the CQL data model, which hides certain implementation details in order to have a more understandable data model. Cassandra is sparse, which means that only data that is used is actually stored. You can visualize this by adding in some test data to Cassandra through CQL.

cqlsh> CREATE KEYSPACE test WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1 } ;
cqlsh> use test ;
cqlsh:test> CREATE TABLE foo (name text, age int, pet text, primary key (name)) ;
cqlsh:test> insert into foo (name, age, pet) values ('yves', 81, 'german shepherd') ;
cqlsh:test> insert into foo (name, pet) values ('coco', 'ferret') ;

cqlsh:test> SELECT * FROM foo ;

name | age  | pet
-----+-----+------------------
coco | null | ferret
yves |  81  | german shepherd

So even it appears that there is a null value, the actual value is nonexistent -- CQL is showing you a null because this makes more sense, intuitively.

If you take a look at the table from the Thrift side, you can see that the table contains no such value for coco's age.

$ bin/cassandra-cli
[default@unknown] use test;
[default@test] list foo;
RowKey: coco
=> (name=, value=, timestamp=1389137986090000)
=> (name=age, value=00000083, timestamp=1389137986090000)
-------------------
RowKey: yves
=> (name=, value=, timestamp=1389137973402000)
=> (name=age, value=00000051, timestamp=1389137973402000)
=> (name=pet, value=6765726d616e207368657068657264, timestamp=1389137973402000)

Here, you can clearly see that yves has two columns: age and pet, while coco only has one: age.