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.company_id = 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 (7, 4, 3, 6) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "grouping_operation": {
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "rows": 13,
            "filtered": 46.154,
            "attached_condition": "(`_testcscart`.`cscart_products_categories`.`product_id` in (7,4,3,6))"
          }
        },
        {
          "table": {
            "table_name": "cscart_categories",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "_testcscart.cscart_products_categories.category_id"
            ],
            "rows": 1,
            "filtered": 100,
            "attached_condition": "((`_testcscart`.`cscart_categories`.`company_id` = 1) and ((`_testcscart`.`cscart_categories`.`usergroup_ids` = '') or find_in_set(0,`_testcscart`.`cscart_categories`.`usergroup_ids`) or find_in_set(1,`_testcscart`.`cscart_categories`.`usergroup_ids`)) and (`_testcscart`.`cscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}