MongoDB Learning-Advanced query [aggregation Group]

Group It takes about a few parameters.

1. Key: The field used to group the document. And Keyf both must have a
2. KEYF: You can accept a JavaScript function. The field used to dynamically determine the grouped document. And key both must have a
3. Initialization of variables used in Initial:reduce
4. Reduce: The reduce function that is executed. The function needs to return a value.
5. Cond: The condition to perform the filtering.
6. Finallize: At reduce execution completion, the result set returns the function that was eventually executed against the result set. An optional

Test data

    Test Data    Db.stocks.insert ({"Day": "2010/10/03", "Time": "2010/10/03 03:47:01 GMT-400", "Price": 4.23});      Db.stocks.insert ({"Day": "2010/10/04", "Time": "2010/10/04 11:28:39 GMT-400", "Price": 4.27});      Db.stocks.insert ({"Day": "2010/10/03", "Time": "2010/10/03 05:00:23 GMT-400", "Price": 4.10});      Db.stocks.insert ({"Day": "2010/10/06", "Time": "2010/10/06 05:27:58 GMT-400", "Price": 4.30});      Db.stocks.insert ({"Day": "2010/10/04", "Time": "2010/10/04 08:34:50 GMT-400", "Price": 4.01});  

Test the Group program code to be executed

Db.runcommand ({"group": {      "ns": "Stocks", "      Key": "Day",      "initial": {"Time": 0},      "$reduce": function ( Doc, prev) {          if (Doc.time > Prev.time) {              prev.price = Doc.price;              Prev.time = Doc.time;          }      }  

Results after the execution

    {"          retval": [              {                  "time": 0              }          ],          "Count": 5,          "keys": 1,          "OK": 1      }  

MongoDB Learning-Advanced query [aggregation Group]