$cond (aggregation)

Definition

$cond: Evaluates a boolean expression to return one of the two specified return expressions.

Compatibility

You can use $cond for deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The $cond expression has one of two syntaxes:

{ $cond: { if: <boolean-expression>, then: <true-case>, else: <false-case> } }

Or:

{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }

$cond requires all three arguments (if-then-else) for either syntax.

If the <boolean-expression> evaluates to true, then $cond evaluates and returns the value of the <true-case> expression. Otherwise, $cond evaluates and returns the value of the <false-case> expression.

The arguments can be any valid expression. For more information on expressions, see Expressions.

Tip

$switch

Example

The following example use a inventory collection with the following documents:

db.inventory.insertMany( [
   { _id: 1, item: "abc1", qty: 300 },
   { _id: 2, item: "abc2", qty: 200 },
   { _id: 3, item: "xyz1", qty: 250 }
] )

The following aggregation operation uses the $cond expression to set the discount value to 30 if qty value is greater than or equal to 250 and to 20 if qty value is less than 250:

db.inventory.aggregate(
   [
      {
         $project:
           {
             item: 1,
             discount:
               {
                 $cond: { if: { $gte: [ "$qty", 250 ] }, then: 30, else: 20 }
               }
           }
      }
   ]
)

The operation returns the following results:

{ _id: 1, item: "abc1", discount: 30 }
{ _id: 2, item: "abc2", discount: 20 }
{ _id: 3, item: "xyz1", discount: 30 }

The following operation uses the array syntax of the $cond expression and returns the same results:

db.inventory.aggregate(
   [
      {
         $project:
           {
             item: 1,
             discount:
               {
                 $cond: [ { $gte: [ "$qty", 250 ] }, 30, 20 ]
               }
           }
      }
   ]
)

Back

$concatArrays

$convert