为此,请使用aggregate()并与$group一起使用。让我们创建一个包含文档的集合-
> db.demo354.insertOne({"Name1":"Chris","Name2":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685a6f8647eb59e5620c0") } > db.demo354.insertOne({"Name1":"Chris","Name2":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685a9f8647eb59e5620c1") } > db.demo354.insertOne({"Name1":"Chris","Name2":"Bob"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685aff8647eb59e5620c2") } > db.demo354.insertOne({"Name1":"Carol","Name2":"John"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685c4f8647eb59e5620c3") } > db.demo354.insertOne({"Name1":"Carol","Name2":"John"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685c5f8647eb59e5620c4") }
在find()方法的帮助下显示集合中的所有文档-
> db.demo354.find();
这将产生以下输出-
{ "_id" : ObjectId("5e5685a6f8647eb59e5620c0"), "Name1" : "Chris", "Name2" : "David" } { "_id" : ObjectId("5e5685a9f8647eb59e5620c1"), "Name1" : "Chris", "Name2" : "David" } { "_id" : ObjectId("5e5685aff8647eb59e5620c2"), "Name1" : "Chris", "Name2" : "Bob" } { "_id" : ObjectId("5e5685c4f8647eb59e5620c3"), "Name1" : "Carol", "Name2" : "John" } { "_id" : ObjectId("5e5685c5f8647eb59e5620c4"), "Name1" : "Carol", "Name2" : "John" }
以下是用于计算由另一个字段分组的唯一字段的数量的查询-
> db.demo354.aggregate([ ... { ... $group: { ... _id: { ... "Name1": "$Name1", ... "Name2": "$Name2" ... }, ... } ... }, ... { ... $group: { ... _id: "$_id.Name1", ... "Name2": { $sum: 1 } ... } ... }, ... { ... $project: { ... _id: 0, ... Name1: "$_id", ... Name2: 1 ... } ... } ... ])
这将产生以下输出-
{ "Name2" : 2, "Name1" : "Chris" } { "Name2" : 1, "Name1" : "Carol" }