Please post your complete DDL and indicate the version of Phoenix and HBase you’re using. Your example should work as expected barring declaration of the table as immutable or COL2 being part of the primary key.
Could you please clarify, how I can set a value to NULL?
After upsert into temp.table (ROWKEY, COL1, COL2) values (100, "ABC", null); the value of COL2 still has a previous value (COL1 has "ABC" as expected).
Or there is only one way - to set STORE_NULLS = true?