查找 MongoDB 中没有特定字段的项目？
要查找没有特定字段的项目，请使用$exists运算符。语法如下-
> db.yourCollectionName.find({"yourItemName":{$exists:false}}).pretty();为了理解语法，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.findDocumentDoNotHaveCertainFields.insertOne({"UserId":101,"UserName":"John","UserAge":21});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a95fb6cea1f28b7aa07fb")
}
> db.findDocumentDoNotHaveCertainFields.insertOne({"UserName":"David","UserAge":22,"UserFavouriteSubject":["C","Java"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a96116cea1f28b7aa07fc")
}
> db.findDocumentDoNotHaveCertainFields.insertOne({"UserName":"Bob","UserAge":24,"UserFavouriteSubject":["MongoDB","MySQL"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a96306cea1f28b7aa07fd")
}在find()方法的帮助下显示集合中的所有文档。查询如下-
> db.findDocumentDoNotHaveCertainFields.find().pretty();
以下是输出-
{
"_id" : ObjectId("5c8a95fb6cea1f28b7aa07fb"),
"UserId" : 101,
"UserName" : "John",
"UserAge" : 21
}
{
"_id" : ObjectId("5c8a96116cea1f28b7aa07fc"),
"UserName" : "David",
"UserAge" : 22,
"UserFavouriteSubject" : [
"C",
"Java"
]
}
{
"_id" : ObjectId("5c8a96306cea1f28b7aa07fd"),
"UserName" : "Bob",
"UserAge" : 24,
"UserFavouriteSubject" : [
"MongoDB",
"MySQL"
]
}这是查找没有特定字段的项目的查询，即UserFavouriteSubject-
> db.findDocumentDoNotHaveCertainFields.find({"UserFavouriteSubject":{$exists:false}}).pretty();以下是输出-
{
"_id" : ObjectId("5c8a95fb6cea1f28b7aa07fb"),
"UserId" : 101,
"UserName" : "John",
"UserAge" : 21
}