Datastax’ CQL documentation gives the impression that INSERT and UPDATE statements are identical, but in fact they are not.

insert_versus_update.cql

CREATE KEYSPACE IF NOT EXISTS test
WITH replication = {'class':'SimpleStrategy', 'replication_factor' : 1};

USE test;

CREATE TABLE IF NOT EXISTS test (
  id int,
  data text,
  PRIMARY KEY (id)
);

//---- Create an entry via UPDATE and update with null value ----
UPDATE test SET data = 'hello world' WHERE id=1;
SELECT * FROM test;
/*
Query returns expected result:
 id | data
----+-------------
  1 | hello world
*/

UPDATE test SET data = null WHERE id=1;
SELECT * FROM test;
/*
Returns zero rows!!!
(0 rows)
*/

UPDATE test SET data='hello world' WHERE id=1 IF EXISTS;
/*
Yup, C* doesn't know this row any more
 [applied]
-----------
     False
*/

//---- Create an entry via INSERT and update with null value ----
INSERT INTO test (id, data) VALUES (2, 'hello');
UPDATE test SET data=null WHERE id=2;
SELECT * FROM test;
/*
Row is still present:
 id | data
----+------
  2 | null

(1 rows)
*/

DROP KEYSPACE IF EXISTS test;