Question

can someone please explain what is meant by:

:new and :old in trigger code.

Answer 1

:new and :old are pseudo-records that let you access the new and old values of particular columns. If I have a table

CREATE TABLE foo (
  foo_id NUMBER PRIMARY KEY,
  bar    VARCHAR2(10),
  baz    VARCHAR2(10)
);

and I insert a row

INSERT INTO foo( foo_id, bar, baz ) 
  VALUES( 1, 'Bar 1', 'Baz 1' );

then in a row-level before insert trigger

:new.foo_id will be 1
:new.bar will be 'Bar 1'
:new.baz will be 'Baz 1'

while

:old.foo_id will be NULL
:old.bar will be NULL
:old.baz will be NULL

If you then update that row

UPDATE foo
   SET baz = 'Baz 2'
 WHERE foo_id = 1

then in a before update row-level trigger

:new.foo_id will be 1
:new.bar will be 'Bar 1'
:new.baz will be 'Baz 2'

while

:old.foo_id will be 1
:old.bar will be 'Bar 1'
:old.baz will be 'Baz 1'

If I then delete the row

DELETE FROM foo
 WHERE foo_id = 1

then in a before delete row-level trigger,

:new.foo_id will be NULL
:new.bar will be NULL
:new.baz will be NULL

while

:old.foo_id will be 1
:old.bar will be 'Bar 1'
:old.baz will be 'Baz 2'