mongodb中如何实现类似于sql中的groupby？

Question

mongodb中如何实现类似于sql中的groupby？

Answer 1

mongodb中实现sql中德group by可以采用聚合函数aggregate
参考代码：

/**
* select avg(pop) avgCitypop from zipsController group by state
*/
exports.avgGroupByStateCityDemo2Async = function (opts) {
var results = {error_code: -1, error_msg: "error"};
var pipeline =[
{
$group: {
_id: "$state",
avgCitypop: {$avg: "$pop"}
}
}

];

return opts.dbs.csdb.collection("zipsController").aggregateAsync(pipeline)