Select Max() with "group by" in mongodb

Question

Please help me to convert this select sentence to mongodb:

Select Name, Max(Value) From table1 Group By Name 

I read this document: http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Group

but still dont know how to apply Max() method instead SUM() as that document.

Thank you.

Answer 1

I have created Mongo Collection as follows.

{ "_id" : ObjectId("4fb36bfd3d1c88bfa15103b1"), "name" : "bob", "value" : 5 } { "_id" : ObjectId("4fb36c033d1c88bfa15103b2"), "name" : "bob", "value" : 3 } { "_id" : ObjectId("4fb36c063d1c88bfa15103b3"), "name" : "bob", "value" : 7 } { "_id" : ObjectId("4fb36c0c3d1c88bfa15103b4"), "name" : "john", "value" : 2 } { "_id" : ObjectId("4fb36c103d1c88bfa15103b5"), "name" : "john", "value" : 4 } { "_id" : ObjectId("4fb36c143d1c88bfa15103b6"), "name" : "john", "value" : 8 } { "_id" : ObjectId("4fb36c163d1c88bfa15103b7"), "name" : "john", "value" : 6 } 

Then by using the following code I group it by their name and max(value)

db.table1.group(     {key: {name:true},         reduce: function(obj,prev) {              if (prev.maxValue < obj.value) {                  prev.maxValue = obj.value;              }           },     initial: { maxValue: 0 }} ); 

The result is shown as

[     {         "name" : "bob",         "maxValue" : 7     },     {         "name" : "john",         "maxValue" : 8     } ] 

It is much simpler with the aggregation framework. You can get the same result with the following code by using aggregation framework.

db.table1.aggregate([     {        $group:{_id:"$name", "maxValue": {$max:"$value"}}     } ]);