Deletion of child records linked to parent records to be deleted etc..

Question

'Combined with WHEN OTHERS, SQLCODE provides a way for you to handle different, specific exceptions without having to use EXCEPTION_INIT pragma. In the next example, I trap two parent child exceptions, -2292 and -2291, and, then take an action appropriate to each situation:
PROCEDURE delete_company (company_id_in IN NUMBER)
IS
BEGIN
DELETE FROM company where company_id = company_id_in;
EXCEPTION
WHEN OTHERS THEN
DECLARE
error_code NUMBER := SQLCODE;
error_msg VARCHAR2(512) := SQLERM;
BEGIN
IF error_code = -2292 THEN
/*Child records found. Delete these too */
DELETE FROM employee WHERE company_id = company_id_in;
/* Now delete parent again */
DELETE FROM company WHERE company_id = company_id_in;
ELSIF error_code = -2291 THEN
/* Parent key not found */
DBMS_OUTPUT_PUT_LINE('Invalid company id' || TO_CHAR(company_id_in));
ELSE
/* This is like a WHEN OTHERS inside a WHEN OTHERS */
DBMS_OUTPUT_PUT_LINE('Error deleting company, error: ' || error_msg
END IF;
END;
END delete_company;
Sourced from Oracle PL/SQL Programming (3rd edition) P.143
1. Are error_code and error_msg assigned automatically or do I have to include assignments? If yes, how?
2. Where do I get a list of commonly caught Oracle error codes and similar code snippets used for business logic?
3. How would you deal with the situation where you detect a parent while you are deleting a child record? Just warn the user and log it hoping the DBA will review the logs?
4. I work within a project where I could not query any parent-child relationships, using, amongst other queries the following query, and, this means that the programming team might not be creating these relationships within the database (Oracle 11g).
What are the technical risks?
Could I be omitting something, because it seems quite incredible to me that parent-child relationships are not in place?
SELECT a.table_name,
a.column_name,
a.constraint_name,
c.owner
FROM ALL_CONS_COLUMNS A, ALL_CONSTRAINTS C
where A.CONSTRAINT_NAME = C.CONSTRAINT_NAME
and a.table_name=:TableName
and C.CONSTRAINT_TYPE = 'R'
Sourced from http://stackoverflow.com/questions/1729996/list-of-foreign-keys-and-the-tables-they-reference

Answer 1

This is bad code, very very bad code. Throw it away and start with a clear idea of what you want to do.
Why is this code very very bad? Because it uses WHEN OTHERS not followed by a RAISE.
Why is this code bad code? Because it uses one WHEN clause instead of several.
Here is an example for handling multiple exceptions:
http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/errors.htm#BABFBHGA
If your error code does not have a predefined name, define the name yourself:
http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/errors.htm#BABIIEFC
If you keep the WHEN OTHERS at the end, be sure to add RAISE so the exception still exists. If you don't, Oracle will not do the automatic rollback it should be doing.
To answer your question about parent-child relationships, it is very important to have them. They stop you deleting a parent that has children, but they do not stop you from deleting children.