Hello everyone!
I want to create a view like this:

create view a(f1, f2, f3, ...) as
(select a.f1, b.f2, NULL, ...
from table1 a, table2 b, ...)

But DB2 raised the error:
SQL0206N "NULL" is not valid in the context where it is used. SQLSTATE=42703

why do you want to select a null value from a table
select '' from table --- would also work

try:

create view a(f1, f2, f3, ...) as
(select a.f1, b.f2, NULLIF(1,1), ...
from table1 a, table2 b, ...)

The only reason that you cannot just SELECT NULL is that NULL has no datatype and every column must have a datatype.

NULLIF(1,1) returns NULL of datatype INT (since 1 is an INT constant);
likely, NULLIF('','') returns NULL of datatype VARCHAR.

For a more generic way of having NULL of a certain datatype (like e.g. DATE) you will have to use CAST:
(or replace "DATE" by any datatype you want).

Yes, Peter.Vanroose is right! I just test, it's ok! Thank you for your help!