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This allows to easily and efficiently dropping records if the current users user does not belongs belong to the group.

SELECT 
    * 
FROM 
  USERGROUPS
    JOIN 
  <ANY ZEPHYR TABLE HERE> 
WHERE
 GROUPNAME = 'zephyr-testers'
    AND
<OTHER ZEPHYR CONDITONS HERE>

If the current user does not belong to the group, then the USERGROUPS table will return no records and the rest of the query will be ignored. Otherwise, the rest of the Zephyr tables will be invoked.

Similarly, you can check global, project and issue permissions.