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$cond (aggregation)

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Definition

$cond

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

The $cond expression has one of two syntaxes:

New in version 2.6.

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

Or:

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

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.

Example

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

{ "_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 ]
               }
           }
      }
   ]
)