JIYIK CN >

Sometimes, we need to find a document by comparing two fields. For example, we may need to consider both the sequence number and the ID number when selecting documents in a MongoDB collection; we will see this later in this tutorial.

In this article, we will see how to compare two fields in MongoDB. Also, we will see a relevant example and explanation to make the topic easier to understand.

Comparing fields in MongoDB

There are two techniques we can use for this purpose. We will use the built-in methods for both techniques find()which are used to search or extract documents from a MongoDB collection.

In our first technique, we will use the keyword $where which primarily matches a specific condition. In our second technique, we will use a simple aggregation with the keyword $expr which allows you to implement an aggregation expression as a parameter.

Let's look at an example to make it clearer.

Example of comparing two fields in MongoDB

In the following example, we will see how to compare two fields in MongoDB. We will compare the sl and id fields.

Now, the command for this purpose is:

Use the $where keyword:

db.mycolletion.find( { $where : "this.sl < this.id" } );

Use the $expr keyword:

db.mycolletion.find({$expr:{$lt:["$sl", "$id"]}})

In the above command, $lt is a comparison operator. You can use some other comparison operators as needed.

Comparison operators available in MongoDB

Below, we share the available comparison operators for aggregation in MongoDB.

• $eq − This stands for the equality operator.
• $ne − This represents the not equal to operator.
• $gt − This stands for greater than operator.
• $gte − This stands for greater than or equal to operator.
• $lt − This stands for the less than operator.
• $lte − This stands for less than or equal to operator.
• $in − This matches any value in the array.
• $nin - This does not match any value in the array.

Now, the two commands shared above will give you the same results as the following command:

{ _id: ObjectId("6371fd850f19826ee6ca5138"),
  sl: 0,
  Name: 'Alen',
  id: '3'
}

请注意The commands shown in this article are for MongoDB databases and need to be run on the MongoDB console.