mongodb提供了聚合函數完成類似的sum函數的功能
比如：$sum
案例：
/**
* select sum(pop) total from zipsController group by state
*/
exports.sumGroupByStateDemoAsync = function (opts) {
var results = {error_code: -1, error_msg: "error"};

var pipeline =[
{
$group: {
_id: "$state",
totalpop: {$sum: "$pop"}
}
}
];

return opts.dbs.csdb.collection("zipsController").aggregateAsync(pipeline)
.timeout(opts.configs.timeoutnormal)
.then(function (value) {
if (value) {
results.error_code = 0;
results.error_msg = "ok";
results.accdetail = value;
} else {//可選
results.error_code = 20001;
results.error_msg = "數據不存在";
results.accdetail = value;
}
return results;
});
}