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                ## $group * 將集合中的文檔分組,可用于統計結果 * _id表示分組的依據,使用某個字段的格式為'$字段' * 例1:統計男生、女生的總人數 ~~~ db.stu.aggregate([ {$group: { _id:'$gender', counter:{$sum:1} } } ]) ~~~ ## Group by null * 將集合中所有文檔分為一組 * 例2:求學生總人數、平均年齡 ~~~ db.stu.aggregate([ {$group: { _id:null, counter:{$sum:1}, avgAge:{$avg:'$age'} } } ]) ~~~ ## 透視數據 * 例3:統計學生性別及學生姓名 ~~~ db.stu.aggregate([ {$group: { _id:'$gender', name:{$push:'$name'} } } ]) ~~~ * 使用$$ROOT可以將文檔內容加入到結果集的數組中,代碼如下 ~~~ db.stu.aggregate([ {$group: { _id:'$gender', name:{$push:'$$ROOT'} } } ]) ~~~
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