Comparison/Sort Order

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Numeric Types

Strings
Arrays
Objects
Dates and Timestamps
Non-existent Fields
BinData

When comparing values of different BSON types in sort operations, MongoDB uses the following comparison order, from lowest to highest:

MinKey (internal type)
Null
Numbers (ints, longs, doubles, decimals)
Symbol, String
Object
Array
BinData
ObjectId
Boolean
Date
Timestamp
Regular Expression
JavaScript Code
MaxKey (internal type)

Note

Range query operators perform comparisons only on fields where the BSON type matches the query value's type. MongoDB enforces comparisons with Comparison Query Operators only on documents where the BSON type of the target field matches the query operand type through Type Bracketing.

Numeric Types

MongoDB treats some types as equivalent for comparison purposes. For instance, numeric types undergo conversion before comparison.

Strings

Binary Comparison

By default, MongoDB uses the simple binary comparison to compare strings.

Collation

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.

Collation specification has the following syntax:

{
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

When specifying collation, the locale field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document.

If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons.

Arrays

In array comparisons:

An ascending sort compares the smallest elements of the array according to the BSON type sort order.
A descending sort compares the largest elements of the array according to the reverse BSON type sort order.
Comparison Query Operators, such as $lt and $gt, perform comparisons on arrays lexicographically.
When comparing a field whose value is a one element array (for example, [ 1 ]) with non-array fields (for example, 2), the comparison is for 1 and 2.
A comparison of an empty array (for example, [ ]) considers the empty array as less than a null value or a missing field value.
A comparison of a nested array (for example, [[1, 2], [3, 4]]) compares any array after the outmost array lexicographically.

Note

Comparison Query Operators enforce type-bracketing when the query is an array. If the indexed value is an array, the operator performs a type-bracketed comparison element-wise over the indexed array.

Objects

MongoDB's comparison of BSON objects uses the following order:

Recursively compare key-value pairs in the order that they appear within the BSON object.
Compare the field types. MongoDB uses the following comparison order for field types, from lowest to highest:
1. MinKey (internal type)
2. Null
3. Numbers (ints, longs, doubles, decimals)
4. Symbol, String
5. Object
6. Array
7. BinData
8. ObjectId
9. Boolean
10. Date
11. Timestamp
12. Regular Expression
13. JavaScript Code
14. MaxKey (internal type)
If the field types are equal, compare the key field names.
If the key field names are equal, compare the field values.
If the field values are equal, compare the next key/value pair (return to step 1). An object without further pairs is less than an object with further pairs.

Dates and Timestamps

Date objects sort before Timestamp objects.

Non-existent Fields

The comparison treats a non-existent field as if it were null. A sort on the a field in documents { } and { a: null } would treat the documents as equivalent in sort order.

BinData

MongoDB sorts BinData in the following order:

First, the length or size of the data.
Then, by the BSON one-byte subtype.
Finally, by the data, performing a byte-by-byte comparison on unsigned bytes.

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