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 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  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') 
WHERE 
  cscart_products_categories.product_id IN (
    12551, 12544, 12546, 12547, 12548, 12549, 
    12550, 12555, 12556, 12557, 12558, 
    12559, 12576, 12577, 12578, 9683, 9685, 
    9686, 9687, 9688, 9689, 9690, 12560, 
    12561, 9775, 9776, 9777, 9778, 9779, 
    9780, 9781, 9782, 9783, 9784, 9785, 
    9786, 9787, 9788, 9790, 9791, 9792, 
    9795, 9797, 9798, 9799, 9800, 9801, 
    9802, 9803, 9804, 9805, 9806, 9807, 
    9808, 9809, 9810, 9811, 9812, 9814, 
    9815, 9816, 9817, 9818, 9820
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00089

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "pt"],
      "key": "pt",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 81,
      "filtered": 100,
      "index_condition": "cscart_products_categories.product_id in (12551,12544,12546,12547,12548,12549,12550,12555,12556,12557,12558,12559,12576,12577,12578,9683,9685,9686,9687,9688,9689,9690,12560,12561,9775,9776,9777,9778,9779,9780,9781,9782,9783,9784,9785,9786,9787,9788,9790,9791,9792,9795,9797,9798,9799,9800,9801,9802,9803,9804,9805,9806,9807,9808,9809,9810,9811,9812,9814,9815,9816,9817,9818,9820)"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "ref": ["noframes_angrybeaver.cscart_products_categories.category_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "cscart_categories.storefront_id in (0,1) 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')"
    }
  }
}