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MongoDB Query to select records having a given key?
To select records having a given key, you can use $exists operator. The syntax is as follows −
db.yourCollectionName.find( { yourFieldName: { $exists : true } } ).pretty();To understand the above syntax, let us create a collection with the document. The query to create a collection with a document is as follows −
> db.selectRecordsHavingKeyDemo.insertOne({"StudentName":"John","StudentAge":21,"StudentMathMarks":78});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8b7be780f10143d8431e0f")
}
> db.selectRecordsHavingKeyDemo.insertOne({"StudentName":"Carol","StudentMathMarks":89});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8b7bfc80f10143d8431e10")
}
> db.selectRecordsHavingKeyDemo.insertOne({"StudentName":"Sam","StudentAge":26,"StudentMathMarks":89});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8b7c1280f10143d8431e11")
}
> db.selectRecordsHavingKeyDemo.insertOne({"StudentName":"Sam","StudentMathMarks":98});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8b7c2180f10143d8431e12")
}Display all documents from a collection with the help of find() method. The query is as follows −
> db.selectRecordsHavingKeyDemo.find().pretty();
The following is the output −
{
"_id" : ObjectId("5c8b7be780f10143d8431e0f"),
"StudentName" : "John",
"StudentAge" : 21,
"StudentMathMarks" : 78
}
{
"_id" : ObjectId("5c8b7bfc80f10143d8431e10"),
"StudentName" : "Carol",
"StudentMathMarks" : 89
}
{
"_id" : ObjectId("5c8b7c1280f10143d8431e11"),
"StudentName" : "Sam",
"StudentAge" : 26,
"StudentMathMarks" : 89
}
{
"_id" : ObjectId("5c8b7c2180f10143d8431e12"),
"StudentName" : "Sam",
"StudentMathMarks" : 98
}Here is the query to select records having a given key −
> db.selectRecordsHavingKeyDemo.find( { StudentMathMarks : { $exists : true } } ).pretty();The following is the output −
{
"_id" : ObjectId("5c8b7be780f10143d8431e0f"),
"StudentName" : "John",
"StudentAge" : 21,
"StudentMathMarks" : 78
}
{
"_id" : ObjectId("5c8b7bfc80f10143d8431e10"),
"StudentName" : "Carol",
"StudentMathMarks" : 89
}
{
"_id" : ObjectId("5c8b7c1280f10143d8431e11"),
"StudentName" : "Sam",
"StudentAge" : 26,
"StudentMathMarks" : 89
}
{
"_id" : ObjectId("5c8b7c2180f10143d8431e12"),
"StudentName" : "Sam",
"StudentMathMarks" : 98
}
Updated on: 30-Jul-2019
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