How to declare a variable in mongodb query?

@kadin 

In MongoDB, you can declare a variable using the let keyword in the $let operator within a pipeline stage of an aggregation query. Here is an example:

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db.collection.aggregate([
   {
      $project: {
         totalAmount: {
            $let: {
               vars: {
                  discount: 0.1
               },
               in: {
                  $multiply: [ "$$discount", "$amount" ]
               }
            }
         }
      }
   }
])

The vars field of the $let operator specifies the variables that you want to declare, and the in field specifies the expression that uses these variables. In this example, the discount variable is set to 0.1, and the totalAmount field is calculated by multiplying the amount field by the discount variable.

You can also use the $set operator to assign a value to a field in a document and create a new field if it doesn't exist. Here is an example:

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db.collection.updateMany(
   {},
   {
      $set: {
         totalAmount: {
            $multiply: [ 0.1, "$amount" ]
         }
      }
   }
)

This will update all documents in the collection and set the totalAmount field to the result of multiplying the amount field by 0.1. If the totalAmount field does not exist in a document, it will be created.

@kadin 

In MongoDB query, you don't declare variables directly within the query itself. However, you can use aggregation framework operators like $set or $addFields to create temporary variables within a query.

Here's an example of using the $set operator to declare a variable within a MongoDB query:

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db.collection.aggregate([
  {
    $set: {
      variableName: "variable value"
    }
  }
])

In this example, the $set operator is used to create a variable called "variableName" with the value "variable value". You can then use this variable in subsequent stages of the aggregation pipeline.

Note that MongoDB does not support direct variable declaration within regular find or update queries. The option of declaring a variable is primarily available in the aggregation framework.