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Conditional $first in MongoDB aggregation ignoring NULL?
You can use $match operator under aggregate() to get the first record. Let us first create a collection with documents −
> db.conditionalFirstDemo.insertOne({_id:100,"StudentName":"Chris","StudentSubject":null}); { "acknowledged" : true, "insertedId" : 100 } > db.conditionalFirstDemo.insertOne({_id:101,"StudentName":"Chris","StudentSubject":null}); { "acknowledged" : true, "insertedId" : 101 } >db.conditionalFirstDemo.insertOne({_id:102,"StudentName":"Chris","StudentSubject":"MongoDB"}); { "acknowledged" : true, "insertedId" : 102 } >db.conditionalFirstDemo.insertOne({_id:103,"StudentName":"Chris","StudentSubject":"MongoDB"}); { "acknowledged" : true, "insertedId" : 103 } > db.conditionalFirstDemo.insertOne({_id:104,"StudentName":"Chris","StudentSubject":null}); { "acknowledged" : true, "insertedId" : 104 }
Following is the query to display all documents from a collection with the help of find() method −
> db.conditionalFirstDemo.find();
This will produce the following output −
{ "_id" : 100, "StudentName" : "Chris", "StudentSubject" : null } { "_id" : 101, "StudentName" : "Chris", "StudentSubject" : null } { "_id" : 102, "StudentName" : "Chris", "StudentSubject" : "MongoDB" } { "_id" : 103, "StudentName" : "Chris", "StudentSubject" : "MongoDB" } { "_id" : 104, "StudentName" : "Chris", "StudentSubject" : null }
Following is the query to conditional $first in MongoDB aggregation −
> db.conditionalFirstDemo.aggregate([ { "$match": { "StudentSubject": { "$ne": null } } }, { "$group": { "_id": "$StudentName", "StudentSubject": { "$first": "$StudentSubject" } }} ]);
This will produce the following output −
{ "_id" : "Chris", "StudentSubject" : "MongoDB" }
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