Audit Table Trigger - sql

Question

Trying to create a trigger for populating an audit table with old data before changes are made to the base table BOOKS
CREATE OR REPLACE TRIGGER populate_audit_table
BEFORE INSERT OR UPDATE OF cost, retail OR DELETE
ON books
REFERENCING NEW as new OLD as old
FOR EACH ROW
DECLARE
dml_operation varchar2(1) :=
CASE WHEN UPDATING THEN 'U'
WHEN DELETING THEN 'D'
ELSE 'I'
END;
BEGIN
INSERT INTO cost_retail_history
(isbn
,title
,pubid
,pubdate
,cost
,retail
,discount
,category
,dml_operation)
SELECT
(old.isbn
,old.title
,old.pubid
,old.pubdate
,old.cost
,old.retail
,old.discount
,old.category
,new.dml_operation)
FROM BOOKS;
end;
I get the following errors:
Errors: TRIGGER POPULATE_AUDIT_TABLE
Line/Col: 8/7 PL/SQL: SQL Statement ignored
Line/Col: 20/13 PL/SQL: ORA-00907: missing right parenthesis
I also tried using VALUES on the INSERT INTO and not designating a source table ("FROM BOOKS")
....VALUES
(old.isbn
,old.title
,old.pubid
,old.pubdate
,old.cost
,old.retail
,old.discount
,old.category
,new.dml_operation);
end;
And I get the following errors:
Line/Col: 8/7 PL/SQL: SQL Statement ignored
Line/Col: 27/18 PL/SQL: ORA-00984: column not allowed here
I've read and re-read https://docs.oracle.com/cd/A64702_01/doc/server.805/a58225/ch4a.htm#1997457 so I feel like I'm missing something simple and just need a point in the right direction.
Thanks,
EDIT:
I dropped the dml_operation as this wasn't required and followed the advice below, but I am still getting errors:
Current version:
CREATE OR REPLACE TRIGGER cost_retail_history
BEFORE INSERT
OR UPDATE OF cost, retail
OR DELETE
ON books
REFERENCING NEW as new OLD as old
FOR EACH ROW
BEGIN
CASE
WHEN INSERTING THEN
INSERT INTO cost_retail_history
(isbn
,title
,pubid
,pubdate
,cost
,retail
,discount
,category)
VALUES
(:new.isbn
,:new.title
,:new.pubid
,:new.pubdate
,:new.cost
,:new.retail
,:new.discount
,:new.category)
WHEN UPDATING cost, retail OR DELETING THEN
INSERT INTO cost_retail_history
(isbn
,title
,pubid
,pubdate
,cost
,retail
,discount
,category)
VALUES
(:old.isbn
,:old.title
,:old.pubid
,:old.pubdate
,:old.cost
,:old.retail
,:old.discount
,:old.category);
END CASE;
END;
I'm getting the errors:
Errors: TRIGGER COST_RETAIL_HISTORY
Line/Col: 4/9 PL/SQL: SQL Statement ignored
Line/Col: 22/5 PL/SQL: ORA-00933: SQL command not properly ended
I can't tell exactly what line the error is on as this would indicate it's with:
...OR DELETE
and
... ,:new.title
which does not make sense to me.

Answer 1

If you were doing insert ... select ... then you should not have the parentheses around the column list; i.e.:
INSERT INTO cost_retail_history
(isbn
...
,dml_operation)
SELECT
old.isbn
...
,new.dml_operation
FROM BOOKS;
but you should not be selecting from the table the trigger is against - partly because you'll get a mutating table error, and you are attempting to insert an audit row for every row in the base table; but mostly because you already have all of the information you need.
With the values clase you need to prefix the old/new values with a colon:
....VALUES
(:old.isbn
,:old.title
,:old.pubid
,:old.pubdate
,:old.cost
,:old.retail
,:old.discount
,:old.category
,:new.dml_operation);
end;
Read more in the documentation.
Oracle has built-in audit tools but presumably this is an exercise.
I can't tell exactly what line the error is on as this would indicate...
This can be slightly confusing with triggers as they are a mix of SQL and PL/SQL. Because you are getting PL/SQL errors the line numbering starts from the beginning of the PL/SQL part, so here BEGIN is line 1, INSERT is line 4; WHEN UPDATING... is line 22.
Those new errors are just because you don't have a semicolon at the end of the first INSERT statement:
,:new.discount
,:new.category);
-----------------------^
WHEN UPDATING cost, retail OR DELETING THEN
INSERT INTO cost_retail_history
The error is reported against line 22 because that WHEN is where it's expecting to see the semicolon, and the first point it can see the previous SQL command hasn't been completed. The error against line 4 is saying where that not-ended command started; so you're getting two messages for one actual problem.