实施MongoDB $ exists和$ ne？
$exists用于检查文件是否存在，而$ne用于不相等条件。首先让我们创建一个包含文档的集合-
> db.existsDemo.insertOne({"Name":"Chris","Age":21});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7c3916d78f205348bc650")
}
> db.existsDemo.insertOne({"Name":"","Age":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7c39a6d78f205348bc651")
}
> db.existsDemo.insertOne({"Name":null,"Age":24});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7c3a66d78f205348bc652")
}
> db.existsDemo.insertOne({"Age":23});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7c3c36d78f205348bc653")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.existsDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd7c3916d78f205348bc650"),
"Name" : "Chris",
"Age" : 21
}
{ "_id" : ObjectId("5cd7c39a6d78f205348bc651"), "Name" : "", "Age" : null }
{ "_id" : ObjectId("5cd7c3a66d78f205348bc652"), "Name" : null, "Age" : 24 }
{ "_id" : ObjectId("5cd7c3c36d78f205348bc653"), "Age" : 23 }以下是实现$exists运算符的查询-
> db.existsDemo.find({"$and":[ {"Name":{"$exists":true}}, {"Name":{"$ne": ""}}]});这将产生以下输出-
{ "_id" : ObjectId("5cd7c3916d78f205348bc650"), "Name" : "Chris", "Age" : 21 }
{ "_id" : ObjectId("5cd7c3a66d78f205348bc652"), "Name" : null, "Age" : 24 }