Trigger compilation error

Hi,

I am creating a trigger in DB2 express version.

When i use the following syntax to create trigger

CREATE TRIGGER USER_PK_TRIGGER
BEFORE INSERT On users
REFERENCING NEW As N
FOR EACH ROW
Begin
if((N.user_id is null) or (N.user_id <= 0)) then
set N.user_id = nextval for user_seq;
end if;
End@


I am getting the following error.

DB21034E The command was processed as an SQL statement because it was not a valid Command Line Processor command. During SQL processing it returned: SQL0104N An unexpected token "(" was found following "EACH ROW Begin if(".
Expected tokens may include: "<space>". LINE NUMBER=6. SQLSTATE=42601

But when i change my create trigger syntax to include atomic word it works.

CREATE TRIGGER USER_PK_TRIGGER
BEFORE INSERT On users
REFERENCING NEW As N
FOR EACH ROW
Begin Atomic
if((N.user_id is null) or (N.user_id <= 0)) then
set N.user_id = nextval for user_seq;
end if;
End@

Why do i need to make my trigger atomic? When it commits does it commit everything (in the ongoing transaction) or only the compound statement in the trigger? What is wrong in my first create trigger syntax?

I am new in DB2 and in most of the examples they have shown that something like first syntax should work, i am not able to figure what what is wrong here. Any help to fix this will be highly appreciated.

Thanks in advance.