how to get projections in mongodb with group operator - mysql

Question

I have a table with columns Column1 & Column2 which looks like this
Colum1 Column2
A 1
A 2
A 3
B 2
B 4
B 6
If I perform following SQL in MYSQL
SELECT Column1, Column2, count(*) from Table group by Column1;
Result is
Column1 Column2 Count(*)
A 1 3
B 2 3
I want to execute similar query on MONGODB
I tried
QUERY1: db.table.aggregate({$group: {_id:"$Column1", count:{$sum:1}} })
QUERY2: db.table.aggregate({$project: {column1:1, column2:1}}, {$group: {_id:"$Column1", count:{$sum:1}} })
However the result for Query2 is same as Query1, It seems like you can not populate fields other than mentioned in $group column.
Is there a way to populate other fields in mongodb along with $group operator ?

Answer 1

I'm not sure I read the mysql query correctly, and I don't understand why this is particularly useful, but $first seems to accomplish the same thing.
However, as mentioned in the $first documentation, the outcome depends on the sorting so you should include a sorting criterion.
Data (column names shortened for brevity)
> db.foo.insert({"C1" : "A", "C2" : 1});
> db.foo.insert({"C1" : "A", "C2" : 2});
> db.foo.insert({"C1" : "A", "C2" : 3});
> db.foo.insert({"C1" : "B", "C2" : 2});
> db.foo.insert({"C1" : "B", "C2" : 4});
> db.foo.insert({"C1" : "B", "C2" : 6});
Aggregation Query
> db.foo.aggregate({$group: {_id:"$C1", C2: { $first: "$C2" }, count:{$sum:1}} })
Results
{
"result" : [
{
"_id" : "B",
"C2" : 2,
"count" : 3
},
{
"_id" : "A",
"C2" : 1,
"count" : 3
}
],
"ok" : 1
}