Question

pro_id | Category
--------+---------
  25   | electronics
  27   | Electrical
  25   | household
  27   | electronics
  25   | industrial

Above is my database table. I want to filter the results by category. If “electronics” is the selected category, the results should include product ids 25 and 27. This is easy. But I want to filter by multiple categories. For example if I select “electronics” and “household”, the result should be only product id 25.

Answer 1

See example here: http://sqlfiddle.com/#!2/09028/7

Join your table to itself by pro_id, then filter by categories to select all product having both categories:

SELECT prod1.pro_id
FROM       prod AS prod1
INNER JOIN prod AS prod2 ON prod1.pro_id = prod2.pro_id
WHERE prod1.Category = 'electronics'
  AND prod2.Category = 'household';

Select all product having at least one category:

SELECT prod.pro_id
FROM  prod
WHERE prod.Category = 'electronics'
   OR prod.Category = 'household'
GROUP BY prod.pro_id;