get entries other than the related ones

Two common approaches have already been suggested by @pritaeas (subquery) and @varmadba (outer join). Both find products with no mapping, but there are practical differences to watch for. The safest, null-safe pattern is a correlated NOT EXISTS check; it avoids the NULL-trap of NOT IN and is easy to read and index.

Here is a concise, robust example — pick the exact columns you need instead of *:

SELECT p.product_id, p.name
FROM products p
WHERE NOT EXISTS (
  SELECT 1
  FROM junction_table j
  WHERE j.product_id = p.product_id
);

Performance and correctness tips:

• Ensure junction_table.product_id is indexed. For example, add an index if needed: CREATE INDEX idx_junction_product ON junction_table(product_id);
• Use EXPLAIN to compare plans for the NOT EXISTS and the left-join variants on your data. On large tables the optimizer choice and available indexes determine which is faster.
• Avoid NOT IN if the subquery can ever return NULL values — that makes the predicate behave unexpectedly. If product_id is declared NOT NULL with a foreign key, NOT IN is safer, but NOT EXISTS is still preferable for clarity.
• Prefer listing required columns instead of SELECT * to reduce I/O.

If the goal is "not related to a particular category (or set of categories)," constrain the subquery on category_id accordingly. Also verify referential integrity (no stray NULL keys) before relying on NOT IN logic.