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 (
    9975, 9976, 9977, 9978, 9979, 9876, 9877, 
    9878, 9880, 9881, 9882, 9883, 9885, 
    9886, 9888, 9889, 9890, 9891, 9892, 
    9893, 9894, 9895, 9896, 9897, 9899, 
    9900, 9901, 9903, 9838, 9839, 9840, 
    9841, 9842, 9843, 9844, 9845, 9846, 
    9847, 9848, 9849, 9850, 9851, 9852, 
    9853, 9854, 9855, 9856, 9857, 9858, 
    9859, 9860, 9861, 9862, 9863, 9814, 
    9815, 9816, 9817, 9818, 9820, 9821, 
    9822, 9823, 9824
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00630

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": 64,
      "filtered": 100,
      "index_condition": "cscart_products_categories.product_id in (9975,9976,9977,9978,9979,9876,9877,9878,9880,9881,9882,9883,9885,9886,9888,9889,9890,9891,9892,9893,9894,9895,9896,9897,9899,9900,9901,9903,9838,9839,9840,9841,9842,9843,9844,9845,9846,9847,9848,9849,9850,9851,9852,9853,9854,9855,9856,9857,9858,9859,9860,9861,9862,9863,9814,9815,9816,9817,9818,9820,9821,9822,9823,9824)"
    },
    "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')"
    }
  }
}