There are a few considerations that need to be taken into account when handling nulls in Datastage:

All DSEE data types are nullable

Null fields do not have a value. Enterprise Edition NULL is respresented by a special value outside the range of any legitimate Datastage values

Nulls can be written to nullable columns only (this setting is specified in the column properties)

The Datastage job will abort when a NULL is to be written to a column which does not allow nulls

Nulls can be converted to or from a value. For instance, in a Sequential File stage the nulls need to be handled explicitly. A value which will be written instead of a null needs to be specified.

In a sequential source stage it is possible to specify values that will be converted to NULLs

A stage can ignore Null fields, can trigger an error or other defined action