mongodb count 每个字段/键的不同值的数量 [英] mongodb count num of distinct values per field/key

问题描述

是否有一个查询来计算一个字段在 DB 中包含多少不同的值.

Is there a query for calculating how many distinct values a field contains in DB.

f.e 我有一个国家字段，国家值有 8 种类型(西班牙、英国、法国等)

f.e I have a field for country and there are 8 types of country values (spain, england, france, etc...)

如果有人添加更多带有新国家/地区的文档，我希望查询返回 9.

If someone adds more documents with a new country I would like the query to return 9.

还有比分组计数更简单的方法吗?

Is there easier way then group and count?

推荐答案

MongoDB 有一个 distinct 命令，它返回一个字段的不同值的数组；您可以检查数组的长度以进行计数.

MongoDB has a distinct command which returns an array of distinct values for a field; you can check the length of the array for a count.

有一个shell db.collection.distinct() 助手也是:

There is a shell db.collection.distinct() helper as well:

> db.countries.distinct('country');
[ "Spain", "England", "France", "Australia" ]

> db.countries.distinct('country').length
4

如 MongoDB 文档中所述:

As noted in the MongoDB documentation:

结果不得大于最大 BSON 大小 (16MB).如果您的结果超过最大 BSON 大小，请使用聚合管道使用 $group 运算符检索不同的值，如 使用聚合管道检索不同的值.

Results must not be larger than the maximum BSON size (16MB). If your results exceed the maximum BSON size, use the aggregation pipeline to retrieve distinct values using the $group operator, as described in Retrieve Distinct Values with the Aggregation Pipeline.

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