Filtering results
Introduction
NGSIv2 implements Simple Query Language and Geographical Queries. They
can be used in both synchronous queries (e.g. GET /v2/entities) and
subscription notifications (in the subject.condition.expression field).
Simple Query Language
The Simple Query Language allows to define conditions that entity attributes must match, e.g. attribute "temperature" has to be greater than 40. It can be also used to define conditions on the attribute metadata, e.g. metadata "accuracy" of attribute "metadata" has to be greater than 0.9.
Examples are found in this section of the API walkthrough. The full syntax definition is found in the "Simple Query Language" section of the NGSIv2 specification.
Geographical Queries
Geographical Queries allow to filter by geographical location, e.g. all the entities located closer than 15 km from the center of Madrid. Of course, properly located entities are mandatory.
Both topics (entity location and geographical queries) are dealt in detail in the "Geospacial properties of entities" and "Geographical Queries" sections of the NGSIv2 specification.