Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE dont_store_delimited_strings (customers, products) AS
SELECT 'cust_a', 'prod_a,prod_b,prod_c' FROM DUAL UNION ALL
SELECT 'cust_b', 'prod_b,prod_d,prod_f' FROM DUAL;
2 rows affected

CREATE TABLE really_dont (products_to_remove) AS
SELECT 'prod_b' FROM DUAL UNION ALL
SELECT 'prod_c' FROM DUAL;
2 rows affected

MERGE INTO dont_store_delimited_strings dst
USING (
  WITH indexed_removal (ptr, idx, max_idx) AS (
    SELECT products_to_remove, ROWNUM, COUNT(*) OVER ()
    FROM   really_dont
  ),
  removed (rid, products, lvl, max_lvl) AS (
    SELECT s.ROWID,
           REPLACE(',' || s.products || ',', ',' || i.ptr || ',', ','),
           i.idx,
           i.max_idx
    FROM   dont_store_delimited_strings s
           INNER JOIN indexed_removal i
           ON (i.idx = 1)
  UNION ALL
    SELECT r.rid,
           REPLACE(r.products, ',' || i.ptr || ',', ','),
           i.idx,
           r.max_lvl
    FROM   removed r
           INNER JOIN indexed_removal i
           ON (r.lvl + 1 = i.idx)
  )
  SELECT rid,
         TRIM(BOTH ',' FROM products) AS products
  FROM   removed
  WHERE  lvl = max_lvl
) src
ON (src.rid = dst.ROWID)
WHEN MATCHED THEN
  UPDATE
  SET products = src.products;
2 rows affected

SELECT *
FROM   dont_store_delimited_strings;


CUSTOMERS
PRODUCTS


cust_a
prod_a

cust_b
prod_d,prod_f