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法律研究如何在 MongoDB 中仅当字段存在时应用条件？
你可以为此使用 $or 运算符。我们首先使用文档创建一个集合 -
> 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")
}以下是使用 find() 方法从集合中显示所有文档的查询 -
> db.applyConditionDemo.find().pretty();
这将生成以下输出 -
{
"_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
}以下是仅当字段存在时应用条件的查询 -
> db.applyConditionDemo.find({ $or: [ { StudentAge: { $exists:false } }, { StudentAge:21 } ]}).pretty();这将生成以下输出 -
{
"_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
}
更新于： 2019-07-30
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