JAVA J2EE 4 U

 SELECT 

    db_key,

    db_name,

    ROUND(AVG((completion_time - start_time) * 1440), 2) AS avg_duration_mins,

    COUNT(*) AS backup_count

FROM (

    SELECT 

        db.db_key,

        db.db_name,

        bs.start_time,

        bs.completion_time

    FROM 

        db db

    JOIN 

        dbinc dbi ON db.db_key = dbi.db_key

    JOIN 

        bset bs ON bs.dbinc_key = dbi.dbinc_key

    WHERE 

        bs.start_time IS NOT NULL

        AND bs.completion_time IS NOT NULL

        AND bs.start_time > SYSDATE - 30

)

GROUP BY 

    db_key, db_name

ORDER BY 

    avg_duration_mins DESC;