Cohorts & Audiences are commonly used to enrol users in courses, via the Cohort/Audience sync enrolment method. To identify the Cohorts/Audiences that have been used to enrol learners in course you can use the following query to create a derived table.
select c.id as cohortid, cm.userid as userid, e.courseid as courseid, c.name as cohort FROM mdl_cohort c JOIN mdl_cohort_members cm ON (cm.cohortid = c.id) JOIN mdl_enrol e ON (e.customint1 = c.id)