ORA-04098: Simple trigger is invalid. Why?

Question

There is something wrong with this trigger. But what?

CREATE  TRIGGER MYCOOLTRIGGER
AFTER INSERT ON MYCOOLTABLE
REFERENCING NEW AS newRow
FOR EACH ROW
DECLARE
BEGIN
END  MYCOOLTRIGGER;

SQL Developer output:

Warning: execution completed with warning
TRIGGER MYCOOLTRIGGER Compiled.

Is there any way to get more info on this Warning?

P.S.

This question could use a better title. ;)

Answer 1

Oracle requires that you have something between BEGIN and END.

You can use NULL (a no-op):

CREATE OR REPLACE TRIGGER MYCOOLTRIGGER
AFTER INSERT ON MYCOOLTABLE
REFERENCING NEW AS newRow
FOR EACH ROW
DECLARE
BEGIN
    NULL;
END  MYCOOLTRIGGER;

Answer 2

If you want to see what the errors are:

show errors trigger mycooltrigger;