For DB2 database, using the HASHLOV function we need to enter the NULL value on some fields within a seed list and we have not found a way to do it.
For example, in correspondence with the empty lines within the seed list city_zone.txt FDM should update CITY_ZONE = NULL on the DB, whatever the format (CHAR, DECIMAL, ...).
Even with the FIXED function we haven't found a way to insert the NULL value.
Release : 4.10
Component : Fast Data Masker
This behavior is due to the fact that empty and null are considered differently for DB2 unlike Oracle and other DBs.
There is an option for the requested functionality: if the value is <NULL> then the database column is set to NULL. In particular if you use the FIXED masking function and set PARM1=<NULL> or if the seedlist contains <NULL> on the line.
Instead of using an empty line/string you have to modify the seedlist and replace empty lines with the <NULL> keyword.