Question

I have a drop-down list which is generated based on the following sql query:

 
SELECT * FROM product WHERE 
    product.id NOT IN (SELECT customer_1.product_id FROM customer_1 WHERE (customer_1.product_id != '$x'))
AND product.id NOT IN (SELECT customer_2.product_id FROM customer_2 WHERE (customer_2.product_id != '$x'))
AND product.id NOT IN (SELECT customer_3.product_id FROM customer_3 WHERE (customer_3.product_id != '$x'));
 

The problem that arises here is the execution time. This query on its own takes about 5.3 s. I have a couple of other similar queries on the same page.

My question is: Is there a better and faster way of achieving the same result?

Thank you in advance.

Answer 1

You may get better performance from LEFT JOINs, looking for NULLs on the right side of the join (the customer_* tables). If I understand your goal, this ought to do the job:

SELECT
  products.*
FROM
  products
  LEFT JOIN customer_1 ON products.id = customer_1.product_id
  LEFT JOIN customer_2 ON products.id = customer_2.product_id
  LEFT JOIN customer_3 ON products.id = customer_3.product_id
WHERE
  products.id != '$x'
  AND customer_1.product_id IS NULL
  AND customer_2.product_id IS NULL
  AND customer_3.product_id IS NULL