SELECT 
  parent_id 
FROM 
  cscart_cp_blog_categories 
WHERE 
  category_id IN (3, 4, 5, 6, 7)

Query time 0.00021

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_cp_blog_categories",
      "access_type": "ALL",
      "possible_keys": ["PRIMARY"],
      "rows": 6,
      "filtered": 83.33333588,
      "attached_condition": "cscart_cp_blog_categories.category_id in (3,4,5,6,7)"
    }
  }
}