$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:{ $cond: { if: <boolean-expression>, then: <true-case>, else: <false-case> } } Or:
{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] } $cond
requires all three arguments (if-then-else
) for either syntax.If the
<boolean-expression>
evaluates totrue
, then$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 ] } } } ] )