分组多个字段并获得在MongoDB中分组在一起的重复字段值的计数
为此，请使用MongoDB聚合，并在其中使用$cond。在$cond表达式一个布尔表达式返回两个指定回报表现之一。
首先让我们创建一个包含文档的集合-
> db.demo536.insertOne({"Name1":"Chris","Name2":"David"});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8c843eef4dcbee04fbbc01")
}
> db.demo536.insertOne({"Name1":"David","Name2":"Chris"});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8c843fef4dcbee04fbbc02")
}
> db.demo536.insertOne({"Name1":"Bob","Name2":"Sam"});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8c843fef4dcbee04fbbc03")
}
> db.demo536.insertOne({"Name1":"Chris","Name2":"David"});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8c843fef4dcbee04fbbc04")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo536.find();
这将产生以下输出-
{ "_id" : ObjectId("5e8c843eef4dcbee04fbbc01"), "Name1" : "Chris", "Name2" : "David" }
{ "_id" : ObjectId("5e8c843fef4dcbee04fbbc02"), "Name1" : "David", "Name2" : "Chris" }
{ "_id" : ObjectId("5e8c843fef4dcbee04fbbc03"), "Name1" : "Bob", "Name2" : "Sam" }
{ "_id" : ObjectId("5e8c843fef4dcbee04fbbc04"), "Name1" : "Chris", "Name2" : "David" }以下是将多个字段分组的查询-
> db.demo536.aggregate([
... {
... $project:
... {
... FirstName1:
... {
... $cond: { if: { $gte: [ "$Name1", "$Name2" ] }, then: "$Name2", else: "$Name1" }
... },
... FirstName2:
... {
... $cond: { if: { $lt: [ "$Name1", "$Name2" ] }, then: "$Name2", else: "$Name1" }
... }
... }
... }
... ,{
... $group:
... {
... _id:
... {
... Name1: "$FirstName1",
... Name2: "$FirstName2"
... }
... ,count: { $sum: 1}
... }
... }
... ])这将产生以下输出＆imnus;。
{ "_id" : { "Name1" : "Bob", "Name2" : "Sam" }, "count" : 1 }
{ "_id" : { "Name1" : "Chris", "Name2" : "David" }, "count" : 3 }