发布于 2015-09-14 15:17:12 | 137 次阅读 | 评论: 0 | 来源: 网络整理
Returns the sum of all the values for a specified field in the grouped documents, as in the second use above.
Alternately, if you specify a value as an argument, $sum will increment this field by the specified value for every document in the grouping. Typically, as in the first use above, specify a value of 1 in order to count members of the group.