仅当MongoDB中的字段存在时才如何应用条件？
您可以为此使用$or运算符。首先让我们创建一个包含文档的集合-
> db.applyConditionDemo.insertOne({"StudentName":"Larry","StudentAge":21,"StudentMarks":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80b78623186894665ae36")
}
> db.applyConditionDemo.insertOne({"StudentName":"Sam","StudentAge":23,"StudentMarks":55});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80b87623186894665ae37")
}
> db.applyConditionDemo.insertOne({"StudentName":"David","StudentAge":21,"StudentMarks":65});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80b95623186894665ae38")
}
> db.applyConditionDemo.insertOne({"StudentName":"Carol","StudentAge":24,"StudentMarks":78});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80ba3623186894665ae39")
}
> db.applyConditionDemo.insertOne({"StudentName":"Chris","StudentAge":21,"StudentMarks":88});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80bae623186894665ae3a")
}
> db.applyConditionDemo.insertOne({"StudentName":"Robert","StudentMarks":98});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cb80c3d623186894665ae3b")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.applyConditionDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cb80b78623186894665ae36"),
"StudentName" : "Larry",
"StudentAge" : 21,
"StudentMarks" : 45
}
{
"_id" : ObjectId("5cb80b87623186894665ae37"),
"StudentName" : "Sam",
"StudentAge" : 23,
"StudentMarks" : 55
}
{
"_id" : ObjectId("5cb80b95623186894665ae38"),
"StudentName" : "David",
"StudentAge" : 21,
"StudentMarks" : 65
}
{
"_id" : ObjectId("5cb80ba3623186894665ae39"),
"StudentName" : "Carol",
"StudentAge" : 24,
"StudentMarks" : 78
}
{
"_id" : ObjectId("5cb80bae623186894665ae3a"),
"StudentName" : "Chris",
"StudentAge" : 21,
"StudentMarks" : 88
}
{
"_id" : ObjectId("5cb80c3d623186894665ae3b"),
"StudentName" : "Robert",
"StudentMarks" : 98
}以下是仅在字段存在时才应用条件的查询-
> db.applyConditionDemo.find({ $or: [ { StudentAge: { $exists:false } }, { StudentAge:21 } ]}).pretty();这将产生以下输出-
{
"_id" : ObjectId("5cb80b78623186894665ae36"),
"StudentName" : "Larry",
"StudentAge" : 21,
"StudentMarks" : 45
}
{
"_id" : ObjectId("5cb80b95623186894665ae38"),
"StudentName" : "David",
"StudentAge" : 21,
"StudentMarks" : 65
}
{
"_id" : ObjectId("5cb80bae623186894665ae3a"),
"StudentName" : "Chris",
"StudentAge" : 21,
"StudentMarks" : 88
}
{
"_id" : ObjectId("5cb80c3d623186894665ae3b"),
"StudentName" : "Robert",
"StudentMarks" : 98
}