SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.company_id = 3 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_products_categories AS product_position_source ON cscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 814 
WHERE 
  cscart_products_categories.product_id IN (
    9845, 9853, 9843, 478, 3321, 514, 8295, 
    480, 9851, 9842, 9849, 479
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00128

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories range PRIMARY,pt,product_id_idx,category_id_idx,product_category_idx,category_product_idx pt 3 99 Using index condition
1 SIMPLE product_position_source eq_ref PRIMARY,pt,product_id_idx,category_id_idx,product_category_idx,category_product_idx PRIMARY 6 const,mahm3t_cs443.cscart_products_categories.product_id 1
1 SIMPLE cscart_categories eq_ref PRIMARY,c_status,p_category_id,idx_category_id PRIMARY 3 mahm3t_cs443.cscart_products_categories.category_id 1 Using where

Result

product_id category_ids position
478 814,1005 0
479 814,1006 43610
480 814,1006 43620
514 808,813,814,998,1002 0
3321 814,1001 0
8295 808,813,814,1004,998,999,1000 0
9842 814,1001,1002 43210
9843 814,1001 43200
9845 1001,814 43230
9849 814,1005 0
9851 814,1009 43410
9853 814,1005 43700