$or (aggregation)

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Definition

Behavior
Error Handling
Example

Definition

$or

Evaluates one or more expressions and returns true if any of the expressions are true. Otherwise, $or returns false.

$or has the following syntax:

{ $or: [ <expression1>, <expression2>, ... ] }

For more information on expressions, see Expression Operators.

Behavior

In addition to the false boolean value, $or evaluates as false the following: null, 0, and undefined values. The $or evaluates all other values as true, including non-zero numeric values and arrays.

Example	Result
`{ $or: [ true, false ] }`	`true`
`{ $or: [ [ false ], false ] }`	`true`
`{ $or: [ null, 0, undefined ] }`	`false`
`{ $or: [ ] }`	`false`

Error Handling

To allow the query engine to optimize queries, $or handles errors as follows:

If any expression supplied to $or would cause an error when evaluated alone, the $or containing the expression may cause an error but an error is not guaranteed.
An expression supplied after the first expression supplied to $or may cause an error even if the first expression evaluates to true.

For example, the following query always produces an error if $x is 0:

db.example.find( {
   $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] }
} )

The following query, which contains multiple expressions supplied to $or, may produce an error if there is any document where $x is 0:

db.example.find( {
   $or: [
      { x: { $eq: 0 } },
      { $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] } }
   ]
} )

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 $or operator to determine if qty is greater than 250 or less than 200:

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            result: { $or: [ { $gt: [ "$qty", 250 ] }, { $lt: [ "$qty", 200 ] } ] }
          }
     }
   ]
)

The operation returns the following results:

{ "_id" : 1, "item" : "abc1", "result" : true }
{ "_id" : 2, "item" : "abc2", "result" : false }
{ "_id" : 3, "item" : "xyz1", "result" : false }
{ "_id" : 4, "item" : "VWZ1", "result" : true }
{ "_id" : 5, "item" : "VWZ2", "result" : true }

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