How to generate DELETE statements in PL/SQL, based on the tables FK relations?

Question

Is it possible via script/tool to generate authomatically many delete statements based on the tables fk relations, using Oracle PL/SQL?

In example: I have the table:

Answer 1

This is a great exercise in developing your PL/SQL skills and general Oracle knowledge!

You need to identify all constrained columns in all tables with relations descending from your master table. You can get all the information you need from two views: ALL_CONSTRAINTS and ALL_CONS_COLUMNS. (If all the tables are in the same schema as the user executing the script, you can use USER_CONSTRAINTS and USER_CONS_COLUMNS if you prefer)

This query will find all the foreign key constraints which refer back to a given table (CUSTOMER in this example):

SELECT constraint_name, table_name, constraint_type
  FROM all_constraints
 WHERE constraint_type = 'R'
   AND r_constraint_name IN (SELECT constraint_name
                               FROM all_constraints
                              WHERE constraint_type IN ('P', 'U')
                                AND table_name = 'CUSTOMER');


CONSTRAINT_NAME                C
------------------------------ -
CUSTOMER_FK1                   R
CUSTOMER_FK4                   R
CUSTOMER_FK5                   R
CUSTOMER_FK3                   R
CUSTOMER_FK2                   R

Now, for each of the results from that query, you can use the CONSTRAINT_NAME column to get a table and column name which you can use to write DELETE statements to delete all child rows in all child tables.

This example gets the table and column name for a constraint called CUSTOMER_FK1

SELECT table_name, column_name
  FROM user_cons_columns
 WHERE constraint_name = 'CUSTOMER_FK1'

TABLE_NAME                    COLUMN_NAME                       
----------------------------- ------------------------------------
RESERVATION                   CUSTOMER_UID

So you could do, for example:

DELETE FROM reservation
 WHERE customer_uid = 00153464

or

DELETE FROM reservation
 WHERE customer_uid IN (SELECT customer_uid
                          FROM customer
                         WHERE customer_type = 'X')

But your child tables also have child tables, so of course you will have to delete those child rows (call them grandchild rows) first. Supposing there is a table called reservation_detail which has a foreign key relationship with reservation, your delete command for reservation_detail might look like:

DELETE FROM reservation_detail 
 WHERE reservation_uid in (SELECT reservation_uid     
                             FROM reservation 
                            WHERE customer_uid IN (SELECT customer_uid
                                                     FROM customer
                                                    WHERE customer_type = 'X')

And if reservation_detail also has children... you get the idea. Of course you could use joins instead of nested queries, but the principle is the same: the more levels deep your dependencies go, the more complex your delete commands become.

So now you know how to do it, the challenge is to write a generic PL/SQL script to delete all child rows, grandchild rows, great-grandchild rows ... (ad infinitum) for any given table, from the bottom up. You will have to employ recursion. Should be a fun program to write!

(Last edit: removed the script; see my other answer for the final solution.)