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Aggregation Operators

$eq (aggregation)

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  • Definition
  • Example

Definition

$eq

Compares two values and returns:

  • true when the values are equivalent.

  • false when the values are not equivalent.

The $eq compares both value and type, using the specified BSON comparison order for values of different types.

$eq has the following syntax:

{ $eq: [ <expression1>, <expression2> ] }

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

Example

Consider an inventory collection with the following documents:

{ "_id" : 1, "item" : "abc1", description: "product 1", qty: 300 }
{ "_id" : 2, "item" : "abc2", description: "product 2", qty: 200 }
{ "_id" : 3, "item" : "xyz1", description: "product 3", qty: 250 }
{ "_id" : 4, "item" : "VWZ1", description: "product 4", qty: 300 }
{ "_id" : 5, "item" : "VWZ2", description: "product 5", qty: 180 }

The following operation uses the $eq operator to determine if qty equals 250:

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            qty: 1,
            qtyEq250: { $eq: [ "$qty", 250 ] },
            _id: 0
          }
     }
   ]
)

The operation returns the following results:

{ "item" : "abc1", "qty" : 300, "qtyEq250" : false }
{ "item" : "abc2", "qty" : 200, "qtyEq250" : false }
{ "item" : "xyz1", "qty" : 250, "qtyEq250" : true }
{ "item" : "VWZ1", "qty" : 300, "qtyEq250" : false }
{ "item" : "VWZ2", "qty" : 180, "qtyEq250" : false }

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$documentNumber (aggregation)

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