MongoDB query to select one field if the other is null and the first field if both are not null?

For this, use $ifNull operator

Let us first create a collection with documents −

> dbquerySelectDemoinsertOne({"Value1":10,"Value2":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cefc0ceef71edecf6a1f6b6")
}
> dbquerySelectDemoinsertOne({"Value1":null,"Value2":30});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cefc0d7ef71edecf6a1f6b7")
}
> dbquerySelectDemoinsertOne({"Value1":60,"Value2":40});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cefc0e2ef71edecf6a1f6b8")
}

Display all documents from a collection with the help of find() method −

> dbquerySelectDemofind()pretty();

Output

{
   "_id" : ObjectId("5cefc0ceef71edecf6a1f6b6"),
   "Value1" : 10,
   "Value2" : null
}
{
   "_id" : ObjectId("5cefc0d7ef71edecf6a1f6b7"),
   "Value1" : null,
   "Value2" : 30
}
{
   "_id" : ObjectId("5cefc0e2ef71edecf6a1f6b8"),
   "Value1" : 60,
   "Value2" : 40
}

Following is the query to select one field if other is null −

> dbquerySelectDemoaggregate([
   {
      $project: {
         "Value1": { "$ifNull": [ "$Value1", "$Value2" ] }
      }
   }
]);

Output

{ "_id" : ObjectId("5cefc0ceef71edecf6a1f6b6"), "Value1" : 10 }
{ "_id" : ObjectId("5cefc0d7ef71edecf6a1f6b7"), "Value1" : 30 }
{ "_id" : ObjectId("5cefc0e2ef71edecf6a1f6b8"), "Value1" : 60 }