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$cond (aggregation)

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$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 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
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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 ]
}
}
}
]
)
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