JAVA J2EE 4 U

 SELECT 

    db_name,

    ROUND(SUM(CASE WHEN bs.status = 'A' THEN 1 ELSE 0 END) * 100.0 / COUNT(*), 2) AS arch_backup_success_pct

FROM 

    rc_backup_set bs

WHERE 

    bs.backup_type = 'L'  -- Archive Log

    AND bs.start_time >= SYSDATE - 30

GROUP BY 

    db_name;