SELECT 
    PROCESS_ID AS PROCESS, 
    COUNT(DISTINCT SESSION_ID) AS CNT
FROM 
    (
        SELECT 
            SESSION_ID, 
            PROCESS_ID, 
            LEAD(PROCESS_ID) OVER (PARTITION BY SESSION_ID ORDER BY EX_TIMESTAMP) AS NEXT_STEP
        FROM 
            PROCESS_LOG
    ) AS SessionSteps
WHERE 
    (PROCESS_ID = 'STEP1' AND (NEXT_STEP = 'STEP2' OR NEXT_STEP IS NULL))
    OR (PROCESS_ID = 'STEP2' AND (NEXT_STEP = 'STEP3' OR NEXT_STEP IS NULL))
    OR (PROCESS_ID = 'STEP3' AND (NEXT_STEP = 'STEP4' OR NEXT_STEP IS NULL))
    OR (PROCESS_ID = 'STEP4' AND (NEXT_STEP = 'STEP5' OR NEXT_STEP IS NULL))
    OR (PROCESS_ID = 'STEP5' AND NEXT_STEP IS NULL)
GROUP BY 
    PROCESS_ID
ORDER BY 
    PROCESS_ID;