WITH SUB1 AS (
    SELECT
        SESSION_ID
        , CAST(REPLACE(PROCESS_ID, 'STEP', '') AS INT)                      AS PROCNUM
        , ROW_NUMBER() OVER (PARTITION BY SESSION_ID ORDER BY EX_TIMESTAMP) AS RNUM
    FROM
        PROCESS_LOG
),
SUB2 AS (
    SELECT
        SESSION_ID
        , MIN(RNUM) AS MINRNUM
    FROM
        SUB1
    WHERE
        PROCNUM <> RNUM
    GROUP BY
        SESSION_ID
)
SELECT 
    'STEP' || PROCNUM AS PROCESS
    , COUNT(DISTINCT
        CASE WHEN MINRNUM IS NULL OR MINRNUM > RNUM THEN a.SESSION_ID END
    ) AS CNT
FROM SUB1 a
    LEFT JOIN SUB2 b ON
    a.SESSION_ID = b.SESSION_ID
GROUP BY
    PROCNUM
ORDER BY
    PROCNUM
;