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MongoDB:聚合之累加操作符

MongoDB的聚合之累加操作符。

官网:https://docs.mongodb.com/manual/reference/operator/aggregation/group/#considerations。

累加操作符

感觉这个没有太多可说的,简单翻译一下。

名字 描述
$avg Returns an average of numerical values. Ignores non-numeric values.(返回平均值)
$first Returns a value from the first document for each group. Order is only defined if the documents are in a defined order.(返回第一个)
$last Returns a value from the last document for each group. Order is only defined if the documents are in a defined order.(返回最后一个)
$max Returns the highest expression value for each group.(返回最大值)
$min Returns the lowest expression value for each group.(返回最小值)
$push Returns an array of expression values for each group.
$addToSet Returns an array of unique expression values for each group. Order of the array elements is undefined.()
$stdDevPop Returns the population standard deviation of the input values.
$stdDevSamp Returns the sample standard deviation of the input values.
$sum Returns a sum of numerical values. Ignores non-numeric values.(返回总和)