I have a data set looks as
我的数据集看起来像
{"BrandId":"a","SessionId":100,"UserName":"tom"}
{"BrandId":"a","SessionId":200,"UserName":"tom"}
{"BrandId":"b","SessionId":300,"UserName":"mike"}
{"Bra
阅读背景：
使用Mongo按组计算多个不同的字段
来源：互联网
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