WITH P as (
    SELECT SESSION_ID, MIN(EX_TIMESTAMP) EX_TIMESTAMP FROM PROCESS_LOG GROUP BY SESSION_ID
), P1 as (
    SELECT * 
    FROM PROCESS_LOG PL
    JOIN P ON PL.SESSION_ID = P.SESSION_ID AND PL.EX_TIMESTAMP = P.EX_TIMESTAMP
    WHERE PROCESS_ID = 'STEP1'
), P2 as (
    SELECT * FROM PROCESS_LOG WHERE PROCESS_ID = 'STEP2'
), P3 as (
    SELECT * FROM PROCESS_LOG WHERE PROCESS_ID = 'STEP3'
), P4 as (
    SELECT * FROM PROCESS_LOG WHERE PROCESS_ID = 'STEP4'
), P5 as (
    SELECT * FROM PROCESS_LOG WHERE PROCESS_ID = 'STEP5'
), A2 as (
    SELECT COUNT(P1.SESSION_ID) CNT
    FROM P1
    JOIN P2 ON P1.SESSION_ID = P2.SESSION_ID AND P1.EX_TIMESTAMP < P2.EX_TIMESTAMP
    LEFT JOIN P3 ON P2.SESSION_ID = P3.SESSION_ID
    LEFT JOIN P4 ON P2.SESSION_ID = P4.SESSION_ID
    LEFT JOIN P5 ON P2.SESSION_ID = P5.SESSION_ID
    WHERE 
          (P3.EX_TIMESTAMP IS NULL OR P2.EX_TIMESTAMP < P3.EX_TIMESTAMP)
      AND (P4.EX_TIMESTAMP IS NULL OR P2.EX_TIMESTAMP < P4.EX_TIMESTAMP)
      AND (P5.EX_TIMESTAMP IS NULL OR P2.EX_TIMESTAMP < P5.EX_TIMESTAMP)
), A3 as (
    SELECT COUNT(P1.SESSION_ID) CNT
    FROM P1
    JOIN P2 ON P1.SESSION_ID = P2.SESSION_ID AND P1.EX_TIMESTAMP < P2.EX_TIMESTAMP
    JOIN P3 ON P2.SESSION_ID = P3.SESSION_ID AND P2.EX_TIMESTAMP < P3.EX_TIMESTAMP
    LEFT JOIN P4 ON P3.SESSION_ID = P4.SESSION_ID
    LEFT JOIN P5 ON P3.SESSION_ID = P5.SESSION_ID
    WHERE 
          (P4.EX_TIMESTAMP IS NULL OR P3.EX_TIMESTAMP < P4.EX_TIMESTAMP)
      AND (P5.EX_TIMESTAMP IS NULL OR P3.EX_TIMESTAMP < P5.EX_TIMESTAMP)
), A4 as (
    SELECT COUNT(P1.SESSION_ID) CNT
    FROM P1
    JOIN P2 ON P1.SESSION_ID = P2.SESSION_ID AND P1.EX_TIMESTAMP < P2.EX_TIMESTAMP
    JOIN P3 ON P2.SESSION_ID = P3.SESSION_ID AND P2.EX_TIMESTAMP < P3.EX_TIMESTAMP
    JOIN P4 ON P3.SESSION_ID = P4.SESSION_ID AND P3.EX_TIMESTAMP < P4.EX_TIMESTAMP
    LEFT JOIN P5 ON P4.SESSION_ID = P5.SESSION_ID
    WHERE 
          (P5.EX_TIMESTAMP IS NULL OR P4.EX_TIMESTAMP < P5.EX_TIMESTAMP)
), A5 as (
    SELECT COUNT(P1.SESSION_ID) CNT
    FROM P1
    JOIN P2 ON P1.SESSION_ID = P2.SESSION_ID AND P1.EX_TIMESTAMP < P2.EX_TIMESTAMP
    JOIN P3 ON P2.SESSION_ID = P3.SESSION_ID AND P2.EX_TIMESTAMP < P3.EX_TIMESTAMP
    JOIN P4 ON P3.SESSION_ID = P4.SESSION_ID AND P3.EX_TIMESTAMP < P4.EX_TIMESTAMP
    JOIN P5 ON P4.SESSION_ID = P5.SESSION_ID AND P4.EX_TIMESTAMP < P5.EX_TIMESTAMP
)

      SELECT 'STEP1' PROCESS, (SELECT COUNT(SESSION_ID) FROM P1) CNT
UNION SELECT 'STEP2' PROCESS, (SELECT CNT FROM A2) CNT
UNION SELECT 'STEP3' PROCESS, (SELECT CNT FROM A3) CNT
UNION SELECT 'STEP4' PROCESS, (SELECT CNT FROM A4) CNT
UNION SELECT 'STEP5' PROCESS, (SELECT CNT FROM A5) CNT