SELECT 
  netting_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      netting_products_categories.link_type = "M", 
      CONCAT(
        netting_products_categories.category_id, 
        "M"
      ), 
      netting_products_categories.category_id
    )
  ) AS category_ids 
FROM 
  netting_products_categories 
  INNER JOIN netting_categories ON netting_categories.category_id = netting_products_categories.category_id 
  AND netting_categories.storefront_id IN (0, 1) 
  AND (
    netting_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, netting_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, netting_categories.usergroup_ids
    )
  ) 
  AND netting_categories.status IN ('A', 'H') 
WHERE 
  netting_products_categories.product_id IN (294, 418, 419) 
GROUP BY 
  netting_products_categories.product_id

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "netting_products_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "pt"],
          "key": "pt",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 27,
          "filtered": 100,
          "index_condition": "netting_products_categories.product_id in (294,418,419)"
        }
      },
      {
        "table": {
          "table_name": "netting_categories",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["category_id"],
          "ref": ["pucuda_cscart.netting_products_categories.category_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "netting_categories.storefront_id in (0,1) and (netting_categories.usergroup_ids = '' or find_in_set(0,netting_categories.usergroup_ids) or find_in_set(1,netting_categories.usergroup_ids)) and netting_categories.`status` in ('A','H')"
        }
      }
    ]
  }
}