MongoDB group by Functionalities

Question

In MySQL

select a,b,count(1) as cnt from list group by a, b having cnt > 2;

I have to execute the group by function using h

Answer 1

Depends on the number of your groups, you might find a simpler and faster solution than group or MapReduce by using distinct:

var res = [];
for( var cur_a = db.list.distinct('a'); cur_a.hasNext(); ) {
  var a = cur_a.next();
  for( var cur_b = db.list.distinct('b'); cur_b.hasNext(); ) {
    var b = cur_b.next();
    var cnt = db.list.count({'a':a,'b':b})
    if (cnt > 2)
      res.push({ 'a': a, 'b' : b 'cnt': cnt}
  }
} 

It will be faster if you have indexes on a and b

db.list.ensureIndex({'a':1,'b':1})