Post-insert vs pre-insert

Question

Dear All,
can anybody explain the difference b/w pre-insert and post-insert trigger ?
i need such example which process something that becomes possible in one trigger and no possible in other ?
Regards
Kashif Butt

Answer 1

Here is an example. Please remove any syntax errors before running :-)
create sequence my_seq;
create table table_a
(id number(20) constraint pk_table_a primary key,
text varchar2(100)
create table table_b
(id number(20) constraint pk_table_b primary key,
a_id number(20) constraint fk1_table_b foreign key references table_a(id),
text varchar2(100)
Create a form with a block based on table_a with items "id" and "text"
"id" is on null canvas (not visible), put "text" on a canvas.
pre-insert:
select my_seq.nextval into :table_a.id;
This can't be done in post-insert. Then the insert will fail because it's the primary key and must be "not null"
post-insert:
insert into table_b(id, a_id, text)
values(my_seq.nextval, :table_a.id, 'Inserted automatically by post-insert');
This can't be done in the pre-insert trigger, because the foreign key constraint will require the record in table_a
to be inserted first.