计算所有MongoDB文档中基于数组的字段中的唯一项？
要计算基于数组的字段中的唯一项，请使用$group和aggregate()。让我们创建一个包含文档的集合-
> db.demo493.insertOne({"SubjectName":["MySQL","MongoDB","Java"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849f97b0f3fa88e22790c4")
}
> db.demo493.insertOne({"SubjectName":["C++","MongoDB","C"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849fa4b0f3fa88e22790c5")
}
> db.demo493.insertOne({"SubjectName":["MySQL","MongoDB","C"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849fb2b0f3fa88e22790c6")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo493.find();
这将产生以下输出-
{ "_id" : ObjectId("5e849f97b0f3fa88e22790c4"), "SubjectName" : [ "MySQL", "MongoDB",
"Java" ] }
{ "_id" : ObjectId("5e849fa4b0f3fa88e22790c5"), "SubjectName" : [ "C++", "MongoDB", "C" ] }
{ "_id" : ObjectId("5e849fb2b0f3fa88e22790c6"), "SubjectName" : [ "MySQL", "MongoDB", "C"
] }以下是查询所有文档中基于数组的字段中的唯一项的查询-
> db.demo493.aggregate([
... { $unwind: "$SubjectName" },
... { $group: { _id: "$SubjectName", Frequency: { $sum : 1 } } }
... ]
... );这将产生以下输出-
{ "_id" : "C++", "Frequency" : 1 }
{ "_id" : "C", "Frequency" : 2 }
{ "_id" : "Java", "Frequency" : 1 }
{ "_id" : "MySQL", "Frequency" : 2 }
{ "_id" : "MongoDB", "Frequency" : 3 }