sql - How to output a warning message to the user in APEX -

i'm uploading csv file values during apex data load wizard below table. trigger runs before each update on table1 struggling display htp output user. know how in apex?

table 1 (csv)

mobile,address,product,comments

12312131,50 long street,product1,null

1231231,14 blah place,product2,comment4

trigger

create or replace trigger "table1_t1" before  update on "table1"  each row   begin      if :old.comments not null      htp.prn ('warning: ' || :new.comments || ' replace ' ||          :old.comments);     end if;   end; 

you can not print message trigger. can validation more error , not warning can't continue process.

or can add dynamic action on before page submit popup confirm msg , if user presses ok process continue.