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.
This behavior is due to the fact that empty and null are considered differently for DB2 unlike Oracle and other DBs.
Release : 4.10
Component : Fast Data Masker
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.