官网：https://docs.mongodb.com/manual/reference/operator/aggregation/group/#considerations。
累加操作符
| 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） |