Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (item, loc) AS
  SELECT 'item1', 'loc1' FROM DUAL UNION ALL     
  SELECT 'item1', 'loc4' FROM DUAL UNION ALL     
  SELECT 'item1', 'loc5' FROM DUAL UNION ALL     
  SELECT 'item1', 'loc6' FROM DUAL UNION ALL    
  SELECT 'item1', 'loc7' FROM DUAL UNION ALL     
  SELECT 'item1', 'loc8' FROM DUAL UNION ALL     
  SELECT 'item1', 'loc9' FROM DUAL UNION ALL
  SELECT 'item2', 'loc1' FROM DUAL UNION ALL     
  SELECT 'item2', 'loc2' FROM DUAL UNION ALL    
  SELECT 'item2', 'loc3' FROM DUAL UNION ALL     
  SELECT 'item2', 'loc4' FROM DUAL UNION ALL     
  SELECT 'item2', 'loc6' FROM DUAL UNION ALL    
  SELECT 'item2', 'loc7' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc2' FROM DUAL UNION ALL    
  SELECT 'item3', 'loc3' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc4' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc5' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc6' FROM DUAL UNION ALL    
  SELECT 'item3', 'loc7' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc8' FROM DUAL UNION ALL     
  SELECT 'item3', 'loc9' FROM DUAL
21 rows affected

WITH Sku (Loc) AS (
  SELECT 'loc1' FROM DUAL UNION ALL     
  SELECT 'loc2' FROM DUAL UNION ALL    
  SELECT 'loc3' FROM DUAL UNION ALL     
  SELECT 'loc4' FROM DUAL UNION ALL     
  SELECT 'loc5' FROM DUAL
)
SELECT t.Item, l.Loc
FROM   Sku l     
       LEFT OUTER JOIN table_name t
       PARTITION BY (t.Item)
       ON (l.Loc = t.Loc)     
WHERE  t.loc is NULL; 


ITEM
LOC


item1
loc2

item1
loc3

item2
loc5

item3
loc1