mongodb的aggregate学习之1-pipeline

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想知道某个country和province下userid的数量，相同userid算一个

如果这么写，

{$group:{"_id": {"country": "$country", "province": "$province"},'count': {$sum: 1}}}

返回了

{"result":[{"_id":{"country":"us","province":"shandong"},"count":2},

{"_id":{"country":"cha","province":"beij"},"count":1},

{"_id":{"country":"cha","province":"shandong"},"count":3}]

从结果来看，计算结果是按照country和province分组了，就是同一个country和province下有几条数据，

不是我们想要的结果，再改

[{$group:{"_id": {"country": "$country", "province": "$province", "uid": "$userid"},'count': {$sum: 1}}}]

返回了

{"result":[{"_id":{"country":"cha","province":"beij","uid":"aaa"},"count":1},

          {"_id":{"country":"us","province":"shandong","uid":"aaa"},"count":2},

          {"_id":{"country":"cha","province":"shandong","uid":"bbb"},"count":2},

          {"_id":{"country":"cha","province":"shandong","uid":"aaa"},"count":1}

]}

再改，

[{$group:{"_id": {"country": "$country", "province": "$province", "uid": "$userid"},'count': {$sum: 1}}},{$group:{"_id": {"country": "$_id.country", "province": "$_id.province", "uid": "$_id.uid"},'count': {$sum: 1}}}];

返回了

{"result":[{"_id":{"country":"cha","province":"shandong","uid":"aaa"},"count":1},

         {"_id":{"country":"cha","province":"shandong","uid":"bbb"},"count":1},

         {"_id":{"country":"us","province":"shandong","uid":"aaa"},"count":1},

          {"_id":{"country":"cha","province":"beij","uid":"aaa"},"count":1}

]}

和上一个的结果相比，就是count都变成了1，还不是想要的结果，再改改

[{$group:{"_id": {"country": "$country", "province": "$province", "uid": "$userid"},'count': {$sum: 1}}},{$group:{"_id": {"country": "$_id.country", "province": "$_id.province", /*"uid": "$_id.uid"*/},'count': {$sum: 1}}}];

就是注释了一点点的代码，返回结果

{"result":[{"_id":{"country":"us","province":"shandong"},"count":1},

         {"_id":{"country":"cha","province":"shandong"},"count":2},

          {"_id":{"country":"cha","province":"beij"},"count":1}]}

是想要的结果了

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