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Legal Studies在 MongoDB 中查找没有某个字段的条目？
要查找没有某个字段的条目，请使用 $exists 运算符。语法如下 −
> db.yourCollectionName.find({"yourItemName":{$exists:false}}).pretty();为理解语法，让我们通过文档创建一个集合。创建带文档的集合的查询如下 −
> 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")
}借助 find() 方法从一个集合里展示所有文档。查询如下 −
> db.findDocumentDoNotHaveCertainFields.find().pretty();
输出如下 −
{
"_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"
]
}这里有一个查询，它可以找到没有 UserFavouriteSubject 字段的条目 −
> db.findDocumentDoNotHaveCertainFields.find({"UserFavouriteSubject":{$exists:false}}).pretty();输出如下 −
{
"_id" : ObjectId("5c8a95fb6cea1f28b7aa07fb"),
"UserId" : 101,
"UserName" : "John",
"UserAge" : 21
}
更新于：2019-07-30
129 次浏览
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