SQL & PL/SQL

I am having trouble setting up a trigger to enable me to enforce referential integrity. I am choosing a trigger as I do not believe it is possible to set it up using constraints.

My parent table (HAB)is of the following structure:

H_ID NUMBER PK
H_CODE VARCHAR2
H_DESCRIPTION

The child table (ATT) is as follows

so_id NUMBER PK
COL_NAME VARCHAR2 PK
VALUE VARCHAR

The table ATT contains in some records information that refers to the HAB table, in these instances col_name = 'HAB' (it can also contain many other value that do not refer to the HAB table)
When the COL_NAME = 'HAB' I want to check that the VALUE column contains a value that is present in HAB.H_ID.

Is it possible to do this with a trigger, I can not figure out how to add a where clause so I only apply the trigger when the col_name column = 'HAB'.

Below is my code so far:

CREATE OR REPLACE TRIGGER TRG_BRYO_ATT_HAB
BEFORE INSERT OR UPDATE
OF VALUE
ON SO_BRY_ATT
REFERENCING NEW AS New OLD AS Old
FOR EACH ROW
WHEN
(NEW.VALUE IS NOT NULL)

DECLARE
Dummy INTEGER; -- used for cursor fetch below
Invalid_habitat EXCEPTION;
Valid_habitat EXCEPTION;
Mutating_table EXCEPTION;
PRAGMA EXCEPTION_INIT (Mutating_table, -4091);

CURSOR Dummy_cursor (hc NUMBER) IS
SELECT h_id FROM hab
WHERE h_id = hc
FOR UPDATE OF h_id;


BEGIN

OPEN Dummy_cursor (:New.value);
FETCH Dummy_cursor INTO Dummy;


IF Dummy_cursor%NOTFOUND THEN
RAISE Invalid_hab;
ELSE
RAISE valid_hab;
END IF;
CLOSE Dummy_cursor;
EXCEPTION
WHEN Invalid_hab THEN
CLOSE Dummy_cursor;
Raise_application_error(-20000, 'Invalid hab'
|| ' code' || TO_CHAR(:New.value));
WHEN Valid_hab THEN
CLOSE Dummy_cursor;
WHEN Mutating_table THEN
NULL;

END TRG_BRYO_ATT_HAB;



/

Any guidance on how to do this, or suggestions of better ways to enforce this would be much appreciated.

Thanks

Stef