sql - exists condition in a trigger -

i'm quite new pl/sql, sorry if question obvious

according trigger documentation, there when ( condition ) triggers. wanted use exists condition, requires subquery, however, have following error :

ora-02251

2251. 00000 - "subquery not allowed here"

*cause: subquery not allowed here in statement.

*action: remove subquery statement.

what did miss?

my condition following :

create or replace trigger mytrigger after update of column on this_table  each row  when (new.status = 'approved' , exists ( select * junction_table this_table_id=new.this_table_id , other_table_id = 'something')) declare begin end; 

i want check whether row associated given value, can find in junction table.

i surely in pl/sql part of trigger, :

• it related trigger rather business logic in itself
• i'd understand missed in documentation , why not possible.

if condition might this, i'm interested.

i write conditional element within trigger itself, like

create or replace trigger mytrigger after update of column on this_table  each row  when (new.status = 'approved')  declare begin   if exists (     select * junction_table this_table_id=new.this_table_id , other_table_id = 'something')) ... end;