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.