null is a special value that is often used to represent an unknown value. For example, a user might not be able to know the value of a field and let it be null.
Example:
{ "field": null };
The expected result is:
{ "field": null }
missing indicates that a name-value pair is missing from an object. If a missing name-value pair is accessed, an empty result value is returned by the query.
As neither the data model nor the system enforces homogeneity for datasets or collections, items in a dataset or collection can be of heterogeneous types and so a field can be present in one object and missing in another.
Example:
{ "field": missing };
The expected result is:
{ }
Since a field with value missing means the field is absent, we get an empty object.