MongoDB中的项目字段
要在MongoDB中投影项目字段，请使用$project。让我们创建一个包含文档的集合-
> db.demo439.insertOne(
... {
... "Name" : "Chris",
... "MarksInformation" : {
... "Marks1" : 67,
... "Marks2" :45,
... "Marks3" : 78
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e77833abbc41e36cc3caeab")
}
> db.demo439.insertOne(
... {
... "Name" : "David",
... "MarksInformation" : {
... "Marks1" : 50,
... "Marks2" :57,
... "Marks3" : 68
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e77833abbc41e36cc3caeac")
}
> db.demo439.insertOne(
... {
... "Name" : "Bob",
... "MarksInformation" : {
... "Score1" : 65,
... "Score2" :71,
...
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e77833bbbc41e36cc3caead")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo439.find();
这将产生以下输出-
{ "_id" : ObjectId("5e77833abbc41e36cc3caeab"), "Name" : "Chris", "MarksInformation" : { "Marks1" : 67, "Marks2" : 45, "Marks3" : 78 } }
{ "_id" : ObjectId("5e77833abbc41e36cc3caeac"), "Name" : "David", "MarksInformation" : { "Marks1" : 50, "Marks2" : 57, "Marks3" : 68 } }
{ "_id" : ObjectId("5e77833bbbc41e36cc3caead"), "Name" : "Bob", "MarksInformation" : { "Score1" : 65, "Score2" : 71 } }以下是对项目字段的查询-
> db.demo439.aggregate({
... $project: {
... "Name" : 1,
... "Marks1": { $cond: [ { $eq:[ { $ifNull: [ "$MarksInformation.Marks1", 0 ] }, 0 ] },{ $ifNull: [ "$MarksInformation.Score1", 0 ] }, "$MarksInformation.Marks1" ] },
... "Marks2": { $cond: [ { $eq:[ { $ifNull: [ "$MarksInformation.Marks2", 0 ] }, 0 ] }, { $ifNull: [ "$MarksInformation.Score2", 0 ] }, "$MarksInformation.Marks2" ] },
... "Marks3": { $cond: [ { $eq:[ { $ifNull: [ "$MarksInformation.Marks3", 0] }, 0 ] }, { $ifNull: [ "$MarksInformation.Score3", 0 ] }, "$MarksInformation.Marks3" ] }
... }})这将产生以下输出-
{ "_id" : ObjectId("5e77833abbc41e36cc3caeab"), "Name" : "Chris", "Marks1" : 67, "Marks2" : 45, "Marks3" : 78 }
{ "_id" : ObjectId("5e77833abbc41e36cc3caeac"), "Name" : "David", "Marks1" : 50, "Marks2" : 57, "Marks3" : 68 }
{ "_id" : ObjectId("5e77833bbbc41e36cc3caead"), "Name" : "Bob", "Marks1" : 65, "Marks2" : 71, "Marks3" : 0 }