Dataset definitions written in ehrQL can be run inside different secure
environments, managed by different providers of EHR data.

For each such secure environment, there is a corresponding "backend"
defined in ehrQL. Each ehrQL backend:

• specifies the datasets available inside each secure environment
• does the necessary translation work to allow the same
  dataset definition to run against data modelled in different ways and
  stored in different systems

When writing a dataset definition you don't need to explicitly reference
any particular backend. But, as not every dataset is available in every
backend, the table schema you use to write your dataset
definition will determine which backends it can be run against.

Below are the backends currently supported in ehrQL, together with the
list of table schemas each one supports.

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