How can I rewrite the following in an efficient manner?

 SELECT emplid, bsl_autonumber, bsl_abs_type, manager_id, absv_request_dt,        begin_dt, return_dt, bsl_approver_id, abs_approved_dt,
       bsl_leave_status, duration_days
  FROM ps_bsl_e_absv_hist
UNION
(SELECT emplid, bsl_autonumber, 'Attendance Regularization', manager_id,
        absv_request_dt, date1, date2, bsl_approver_id, abs_approved_dt,
        bsl_leave_status, 1
   FROM ps_bsl_swipecard)
UNION
(SELECT emplid, bsl_autonumber, 'Compensatory OFF', manager_id, request_dt,
        date2, date2, bsl_approver_id, abs_approved_dt, bsl_leave_status, 1
   FROM ps_bsl_compoff_tbl)
UNION
(SELECT emplid, bsl_autonumber,
        DECODE (bsl_abs_type,
                'AR', 'Attendance Regularization',
                'CO', 'Compensatory OFF',