选择2个字段并返回具有其不同值的排序数组 [英] select 2 fields and return a sorted array with their distinct values

问题描述

说我们有以下文档:

{a: 1, b: 2},
{a: 2, b: 0},
{a: 3, b: 1}

我想要一个查询，该查询将返回:

I want a query that will return:

[0, 1, 2, 3]

我想知道是否有比以下方法更快的方法:

I want to know if there is a way to do this faster than:

• 仅进行2个查询，一个选择a，另一个选择b，然后合并到我的应用程序中.
• 使用map reduce(与以前的方法相比，速度很慢)

• just making 2 queries, one selecting a, the other selecting b then merging in my application.
• using map reduce(which was reaaaaly slow compared to the previous method)

推荐答案

您需要 $push 累加器运算符可返回集合中的"a"和"b"数组.

You need to $group our documents and use the $push accumulator operator to return an array of "a" and "b" within the collection.

在 $project 运算符中，您使用 $setUnion 运算符，以过滤出重复项.

In the $project operator you use the $setUnion operator to filter out the duplicates.

db.coll.aggregate(
    [
        { "$group": { 
            "_id": null, 
            "a": { "$push": "$a" }, 
            "b": { "$push": "$b" } 
        }}, 
        { "$project": {
            "_id": 0, 
            "merged": { "$setUnion": [ "$a", "$b" ] } 
        }} 
    ]
)

产生:

{ "merged" : [ 3, 2, 0, 1 ] }

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