Query

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

Results

[
  {
    "category": "Beverages", 
    "cheapestProducts": [
      {
        "category": "Beverages", 
        "unitsInStock": 20, 
        "productName": "Guaran\u00c3\u00a1 Fant\u00c3\u00a1stica", 
        "unitPrice": 4.5, 
        "productId": 24
      }
    ]
  }, 
  {
    "category": "Condiments", 
    "cheapestProducts": [
      {
        "category": "Condiments", 
        "unitsInStock": 13, 
        "productName": "Aniseed Syrup", 
        "unitPrice": 10.0, 
        "productId": 3
      }
    ]
  }, 
  {
    "category": "Produce", 
    "cheapestProducts": [
      {
        "category": "Produce", 
        "unitsInStock": 4, 
        "productName": "Longlife Tofu", 
        "unitPrice": 10.0, 
        "productId": 74
      }
    ]
  }, 
  {
    "category": "Meat/Poultry", 
    "cheapestProducts": [
      {
        "category": "Meat/Poultry", 
        "unitsInStock": 21, 
        "productName": "Tourti\u00c3\u00a8re", 
        "unitPrice": 7.45, 
        "productId": 54
      }
    ]
  }, 
  {
    "category": "Seafood", 
    "cheapestProducts": [
      {
        "category": "Seafood", 
        "unitsInStock": 24, 
        "productName": "Konbu", 
        "unitPrice": 6.0, 
        "productId": 13
      }
    ]
  }, 
  {
    "category": "Dairy Products", 
    "cheapestProducts": [
      {
        "category": "Dairy Products", 
        "unitsInStock": 112, 
        "productName": "Geitost", 
        "unitPrice": 2.5, 
        "productId": 33
      }
    ]
  }, 
  {
    "category": "Confections", 
    "cheapestProducts": [
      {
        "category": "Confections", 
        "unitsInStock": 25, 
        "productName": "Teatime Chocolate Biscuits", 
        "unitPrice": 9.2, 
        "productId": 19
      }
    ]
  }, 
  {
    "category": "Grains/Cereals", 
    "cheapestProducts": [
      {
        "category": "Grains/Cereals", 
        "unitsInStock": 38, 
        "productName": "Filo Mix", 
        "unitPrice": 7.0, 
        "productId": 52
      }
    ]
  }
]