MongoDB相当于SQL COUNT GROUP BY

Question

Been reading the mongodb aggregation and group docs, however still unclear to me.

Consider the collection with docs:

{ a : 1, b : 0 }

{ a : 1, b : 1 }

{ a : 2, b : 2 }

{ a : 3, b : 3 }

I want to count the number of documents for each value of a and sort by the count.

In the relational style it would look something like this:

SELECT COUNT(distinct b) AS total FROM example_table GROUP BY a ORDER BY total DESC;

Any suggestions on how to do this in mongo?

Answer 1

您首先用$sum:{1}在_id:"$a"上的$group （这基本上是您对不同a的计数），然后sort总和进行sort 。