Find duplicate records in MongoDB - mongodb

Question

How would I find duplicate fields in a mongo collection.
I'd like to check if any of the "name" fields are duplicates.
{
"name" : "ksqn291",
"__v" : 0,
"_id" : ObjectId("540f346c3e7fc1054ffa7086"),
"channel" : "Sales"
}
Many thanks!

Answer 1

Use aggregation on name and get name with count > 1:
db.collection.aggregate([
{"$group" : { "_id": "$name", "count": { "$sum": 1 } } },
{"$match": {"_id" :{ "$ne" : null } , "count" : {"$gt": 1} } },
{"$project": {"name" : "$_id", "_id" : 0} }
]);
To sort the results by most to least duplicates:
db.collection.aggregate([
{"$group" : { "_id": "$name", "count": { "$sum": 1 } } },
{"$match": {"_id" :{ "$ne" : null } , "count" : {"$gt": 1} } },
{"$sort": {"count" : -1} },
{"$project": {"name" : "$_id", "_id" : 0} }
]);
To use with another column name than "name", change "$name" to "$column_name"

Answer 2

You can find the list of duplicate names using the following aggregate pipeline:
Group all the records having similar name.
Match those groups having records greater than 1.
Then group again to project all the duplicate names as an array.
The Code:
db.collection.aggregate([
{$group:{"_id":"$name","name":{$first:"$name"},"count":{$sum:1}}},
{$match:{"count":{$gt:1}}},
{$project:{"name":1,"_id":0}},
{$group:{"_id":null,"duplicateNames":{$push:"$name"}}},
{$project:{"_id":0,"duplicateNames":1}}
])
o/p:
{ "duplicateNames" : [ "ksqn291", "ksqn29123213Test" ] }

Answer 3

The answer anhic gave can be very inefficient if you have a large database and the attribute name is present only in some of the documents.
To improve efficiency you can add a $match to the aggregation.
db.collection.aggregate(
{"$match": {"name" :{ "$ne" : null } } },
{"$group" : {"_id": "$name", "count": { "$sum": 1 } } },
{"$match": {"count" : {"$gt": 1} } },
{"$project": {"name" : "$_id", "_id" : 0} }
)

Answer 4

Another option is to use $sortByCount stage.
db.collection.aggregate([
{ $sortByCount: '$name' }
]
This is the combination of $group & $sort.
The $sortByCount stage is equivalent to the following $group + $sort sequence:
{ $group: { _id: <expression>, count: { $sum: 1 } } },
{ $sort: { count: -1 } }

Answer 5

db.getCollection('orders').aggregate([
{$group: {
_id: {name: "$name"},
uniqueIds: {$addToSet: "$_id"},
count: {$sum: 1}
}
},
{$match: {
count: {"$gt": 1}
}
}
])
First Group Query the group according to the fields.
Then we check the unique Id and count it, If count is greater then 1 then the field is duplicate in the entire collection so that thing is to be handle by $match query.

Answer 6

this is how we can achieve this in mongoDB compass

Answer 7

In case you need to see all duplicated rows:
db.collection.aggregate([
{"$group" : { "_id": "$name", "count": { "$sum": 1 },"data": { "$push": "$$ROOT" }}},
{"$unwind": "$data"},
{"$match": {"_id" :{ "$ne" : null } , "count" : {"$gt": 1} } },
]);

Answer 8

If somebody is looking for a query for duplicates with an extra "$and" where clause, like "and where someOtherField is true"
The trick is to start with that other $match, because after the grouping you don't have all the data available anymore
// Do a first match before the grouping
{ $match: { "someOtherField": true }},
{ $group: {
_id: { name: "$name" },
count: { $sum: 1 }
}},
{ $match: { count: { $gte: 2 } }},
I searched for a very long time to find this notation, hope I can help somebody with the same problem

Answer 9

Search for duplicates in Compass Mongo db using $sortByCount
[screenshot]: https://i.stack.imgur.com/L85QV.png

Answer 10

Sometimes you want to find duplicates regardless the case, when you want to create a case insensitive index for instance. In this case you can use this aggregation pipeline
db.collection.aggregate([
{'$group': {'_id': {'$toLower': '$name'}, 'count': { '$sum': 1 }, 'duplicates': { '$push': '$$ROOT' } } },
{'$match': { 'count': { '$gt': 1 } }
]);
Explanation:
group by name but first change the case to lower case and push the docs to the duplicates array.
match those groups having records greater than 1 (the duplicates).