在MongoDB上实现类似于MySQL Union的查询？
aggregate()。让我们创建一个包含文档的集合-> db.demo486.insertOne({_id:1,"Amount":30,"No":4});
{ "acknowledged" : true, "insertedId" : 1 }
> db.demo486.insertOne({_id:2,"Amount":40,"No":2});
{ "acknowledged" : true, "insertedId" : 2 }
> db.demo486.insertOne({_id:3,"Amount":60,"No":6});
{ "acknowledged" : true, "insertedId" : 3 }在find()方法的帮助下显示集合中的所有文档-
> db.demo486.find();
这将产生以下输出-
{ "_id" : 1, "Amount" : 30, "No" : 4 }
{ "_id" : 2, "Amount" : 40, "No" : 2 }
{ "_id" : 3, "Amount" : 60, "No" : 6 }以下是使用文档创建第二个集合的查询-
> db.demo487.insertOne({_id:1,"Price":10,"No":4});
{ "acknowledged" : true, "insertedId" : 1 }
> db.demo487.insertOne({_id:2,"Price":80,"No":9});
{ "acknowledged" : true, "insertedId" : 2 }
> db.demo487.insertOne({_id:3,"Price":20,"No":6});
{ "acknowledged" : true, "insertedId" : 3 }在find()方法的帮助下显示集合中的所有文档-
> db.demo487.find();
这将产生以下输出-
{ "_id" : 1, "Price" : 10, "No" : 4 }
{ "_id" : 2, "Price" : 80, "No" : 9 }
{ "_id" : 3, "Price" : 20, "No" : 6 }以下是在MongoDB中对UNION的两个查询-
> db.getCollection('demo486').aggregate([
... {$lookup : { from : "demo487",localField : "No", foreignField :"No", as:"demo487"}},
... {$unwind : "$demo487"},
... {
... $group : {
... _id : {
... No :"$No",
... },
... TotalValue : { $sum : { $add: [ "$Amount", "$demo487.Price" ] }}
... }
... },
... {$sort : {"_id.No":1}},
... {
... $project : {
... No :"$_id.No",
... TotalValue : 1,
... _id : 0
... }
... }
... ])这将产生以下输出-
{ "TotalValue" : 40, "No" : 4 }
{ "TotalValue" : 80, "No" : 6 }