SELECT u.id, u.picture, u.firstname, targetusers.lastname,.....,
       COALESCE(ul.timeaccess, 0) AS lastaccess,
       ctx.id AS ctxid, ctx.path AS ctxpath, ....
FROM
    (SELECT DISTINCT tu.id
        FROM b1user tu
        JOIN b1user_enrolments ue ON ue.userid = tu.id
        JOIN b1enrol e ON e.id = ue.enrolid AND e.courseid = :o_courseid1
) targetusers
JOIN b1user u ON u.id = targetusers.id
LEFT JOIN b1user_lastaccess ul ON (ul.userid = targetusers.id AND ul.courseid = :o_courseid2)
LEFT JOIN b1context ctx ON (ctx.instanceid = targetusers.id AND ctx.contextlevel = :o_contextlevel)
<<OTHER FILTERS GO HERE>>
ORDER BY xxxxxxx ASC