在MongoDB中获取特定的字段值
要获取特定的字段值，请使用$in运算符。$in选择文档，其中字段的值等于指定数组中的任何值。
首先让我们创建一个包含文档的集合-
> db.indexesDemo.createIndex({"StudentFirstName":1});
{
"createdCollectionAutomatically" : true,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
> db.indexesDemo.insertOne({"StudentFirstName":"John","StudentLastName":"Smith"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06de4d25ddae1f53b621dd")
}
> db.indexesDemo.insertOne({"StudentFirstName":"Chris","StudentLastName":"Brown"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06de5825ddae1f53b621de")
}
> db.indexesDemo.insertOne({"StudentFirstName":"John","StudentLastName":"Doe"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06de6725ddae1f53b621df")
}
> db.indexesDemo.insertOne({"StudentFirstName":"David","StudentLastName":"Miller"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06de7225ddae1f53b621e0")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.indexesDemo.find();
这将产生以下输出-
{ "_id" : ObjectId("5e06de4d25ddae1f53b621dd"), "StudentFirstName" : "John", "StudentLastName" : "Smith" }
{ "_id" : ObjectId("5e06de5825ddae1f53b621de"), "StudentFirstName" : "Chris", "StudentLastName" : "Brown" }
{ "_id" : ObjectId("5e06de6725ddae1f53b621df"), "StudentFirstName" : "John", "StudentLastName" : "Doe" }
{ "_id" : ObjectId("5e06de7225ddae1f53b621e0"), "StudentFirstName" : "David", "StudentLastName" : "Miller" }以下是对索引的查询-
> db.indexesDemo.find(
... { StudentFirstName: { $in: [ "John", "David" ] } },
... { _id: 0, StudentLastName: 0 }
... );这将产生以下输出-
{ "StudentFirstName" : "David" }
{ "StudentFirstName" : "John" }
{ "StudentFirstName" : "John" }