Query

{
  "group": {
    "into": "g", 
    "by": "p.category"
  }, 
  "as": "p", 
  "from": "Products", 
  "select": {
    "category": "g.key", 
    "mostExpensiveProducts": {
      "where": "p.unitPrice == g.max(p.unitPrice)", 
      "as": "p", 
      "from": "g", 
      "select": "p"
    }
  }
}

Results

[
  {
    "category": "Beverages", 
    "mostExpensiveProducts": [
      {
        "category": "Beverages", 
        "unitsInStock": 17, 
        "productName": "C\u00c3\u00b4te de Blaye", 
        "unitPrice": 263.5, 
        "productId": 38
      }
    ]
  }, 
  {
    "category": "Condiments", 
    "mostExpensiveProducts": [
      {
        "category": "Condiments", 
        "unitsInStock": 24, 
        "productName": "Vegie-spread", 
        "unitPrice": 43.9, 
        "productId": 63
      }
    ]
  }, 
  {
    "category": "Produce", 
    "mostExpensiveProducts": [
      {
        "category": "Produce", 
        "unitsInStock": 20, 
        "productName": "Manjimup Dried Apples", 
        "unitPrice": 53.0, 
        "productId": 51
      }
    ]
  }, 
  {
    "category": "Meat/Poultry", 
    "mostExpensiveProducts": [
      {
        "category": "Meat/Poultry", 
        "unitsInStock": 0, 
        "productName": "Th\u00c3\u00bcringer Rostbratwurst", 
        "unitPrice": 123.79, 
        "productId": 29
      }
    ]
  }, 
  {
    "category": "Seafood", 
    "mostExpensiveProducts": [
      {
        "category": "Seafood", 
        "unitsInStock": 42, 
        "productName": "Carnarvon Tigers", 
        "unitPrice": 62.5, 
        "productId": 18
      }
    ]
  }, 
  {
    "category": "Dairy Products", 
    "mostExpensiveProducts": [
      {
        "category": "Dairy Products", 
        "unitsInStock": 79, 
        "productName": "Raclette Courdavault", 
        "unitPrice": 55.0, 
        "productId": 59
      }
    ]
  }, 
  {
    "category": "Confections", 
    "mostExpensiveProducts": [
      {
        "category": "Confections", 
        "unitsInStock": 40, 
        "productName": "Sir Rodney's Marmalade", 
        "unitPrice": 81.0, 
        "productId": 20
      }
    ]
  }, 
  {
    "category": "Grains/Cereals", 
    "mostExpensiveProducts": [
      {
        "category": "Grains/Cereals", 
        "unitsInStock": 2, 
        "productName": "Gnocchi di nonna Alice", 
        "unitPrice": 38.0, 
        "productId": 56
      }
    ]
  }
]