通过使用MongoDB $ group获取值列表？
要获取值列表，请使用$group聚合和$push运算符。首先让我们创建一个包含文档的集合-
> db.groupByDemo.insertOne({"UserName":"John","Subject":"MongoDB"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0457806ebf1256f136")
}
> db.groupByDemo.insertOne({"UserName":"Larry","Subject":"MySQL"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0657806ebf1256f137")
}
> db.groupByDemo.insertOne({"UserName":"John","Subject":"Java"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0d57806ebf1256f138")
}
> db.groupByDemo.insertOne({"UserName":"John","Subject":"C"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f1357806ebf1256f139")
}
> db.groupByDemo.insertOne({"UserName":"Larry","Subject":"SQL Server"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f1c57806ebf1256f13a")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.groupByDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd69f0457806ebf1256f136"),
"UserName" : "John",
"Subject" : "MongoDB"
}
{
"_id" : ObjectId("5cd69f0657806ebf1256f137"),
"UserName" : "Larry",
"Subject" : "MySQL"
}
{
"_id" : ObjectId("5cd69f0d57806ebf1256f138"),
"UserName" : "John",
"Subject" : "Java"
}
{
"_id" : ObjectId("5cd69f1357806ebf1256f139"),
"UserName" : "John",
"Subject" : "C"
}
{
"_id" : ObjectId("5cd69f1c57806ebf1256f13a"),
"UserName" : "Larry",
"Subject" : "SQL Server"
}以下是使用$group和$push获取值列表的查询-
> db.groupByDemo.aggregate({$group: { '_id': '$UserName', 'Subject': { $push: '$Subject'}}});这将产生以下输出-
{ "_id" : "Larry", "Subject" : [ "MySQL", "SQL Server" ] }
{ "_id" : "John", "Subject" : [ "MongoDB", "Java", "C" ] }