Aggregating Time-Series Data - Learning Apache Cassandra - page 227

Counters and upserts
We'll also note that our earlier UPDATE statement attempted to increment a column in a
row that did not yet exist. As we saw in Chapter 7 , Expanding Your Data Model , this is an
upsert operation; the UPDATE statement actually creates the row and increments the values
accordingly. Not surprisingly, if we attempt to increment a counter column that doesn't yet
have a value, its initial value is considered zero.