Use of sequence in trigger

Question

I need to ensure that a unique key value is stored in a column. Values must be persistent across DB restores. A Sequence is fine to initially populate the column, but I must also ensure a key is applied to records inserted later (by third-party code unaware of the key column).
If I try to plant a trigger to set the column value to sequence.nextval, compilation error occurs: "PLS-00357: ... Sequence reference ... not allowed in this context."
Are special tactics needed to allow use of sequences in triggers? Can you suggest another approach to enforce key maintenance?

Answer 1

Log in as scott/tiger and do the following:
CREATE SEQUENCE deptno
START WITH 50
INCREMENT BY 10;
CREATE OR REPLACE FUNCTION next_dept
RETURN NUMBER AS
v_new_dept dept.deptno%TYPE;
BEGIN
SELECT deptno.NEXTVAL
INTO v_new_dept
FROM DUAL;
RETURN v_new_dept;
END;
CREATE OR REPLACE TRIGGER new_dept
BEFORE INSERT ON dept
FOR EACH ROW
BEGIN
:NEW.deptno := next_dept();
-- this will fail when next_dept
-- returns 100 because deptno is
-- only 2 digits.
END;
INSERT INTO dept
-- you must provide a dummy deptno
-- to satisfy the PK constraint
VALUES(0, 'SURFING', 'MAUI');
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