SELECT
    foo.usr_id   AS user,
    foo.sesn_id  AS session1,
    bar.sesn_id  AS session2,
    foo.sesn_bgn AS start1,
    bar.sesn_bgn AS start2,
    foo.sesn_end AS end1,
    bar.sesn_end AS end2
FROM
    sesn foo,
    sesn bar
WHERE
    foo.usr_id = bar.usr_id
    AND bar.sesn_bgn >= foo.sesn_bgn
    AND bar.sesn_bgn <= foo.sesn_end
    AND foo.sesn_id != bar.sesn_id
    -- AND (conditions on min duration etc here)