关于mongodb的aggregate问题，求解答

aggregate是monogodb的聚合函数。完成类似于sql中的sum,count,group by等相关操作
1：count(*)

/**
* select count(*) count from zipsController
*/
var pipeline = [

{
$group: {
_id: null,
count: {$sum: 1}
}
}
];
2:sum(*)
/**
* select sum(pop) total from zipsController
*/
var pipeline = [
{
$group: {
_id: null,
total: {$sum: "$pop"}
}
}
];
3:group by

/**
* select sum(pop) total from zipsController group by state
*/
var pipeline =[
{
$group: {
_id: "$state",
totalpop: {$sum: "$pop"}
}
}
];