The HL7 FHIR and CQL are a powerful duo that pave the way for the next generation of digital healthcare. While the FHIR standard provides the common data model for the healthcare data, the CQL specification defines the language for expressing clinical knowledge in a way that can be executed against the FHIR data model.
As a formal language, the Clinical Quality Language is used to represent clinically-focused logic such as quality measurements and clinical decision support and therefore supports the creation, distribution, execution, and maintenance of computable clinical knowledge.
I've created this online editor to make it easier to work with CQL code. It allows you to create and edit CQL files in the browser and execute their content against FHIR endpoints.