with history AS
        (
        select
        		session_id 
        	,lag(process_id) OVER (partition by session_id order by ex_timestamp) bef
        	,process_id 
        	,lead(process_id, 1) OVER (partition by session_id order by ex_timestamp, session_id) P2
        	,lead(process_id, 2) OVER (partition by session_id order by ex_timestamp, session_id) P3
        	,lead(process_id, 3) OVER (partition by session_id order by ex_timestamp, session_id) P4
        	,lead(process_id, 4) OVER (partition by session_id order by ex_timestamp, session_id) P5
        from
        	process_log
        group by 
        	session_id, process_id
        ), result as (
        select 'STEP1' PROCESS, count(session_id) CNT FROM history where bef is null AND  process_id = 'STEP1'
        UNION ALL
        select 'STEP2' PROCESS, count(session_id) CNT FROM history where bef is null AND process_id = 'STEP1' AND P2 = 'STEP2'
        UNION ALL
        select 'STEP3' PROCESS, count(session_id) CNT FROM history where bef is null AND process_id = 'STEP1' AND P2 = 'STEP2' AND P3 = 'STEP3'
        UNION ALL
        select 'STEP4' PROCESS, count(session_id) CNT FROM history where bef is null AND process_id = 'STEP1' AND P2 = 'STEP2' AND P3 = 'STEP3' AND P4 = 'STEP4'
        UNION ALL
        select 'STEP5' PROCESS, count(session_id) CNT FROM history where bef is null AND process_id = 'STEP1' AND P2 = 'STEP2' AND P3 = 'STEP3' AND P4 = 'STEP4' AND P5 = 'STEP5'
        ) 
        select PROCESS, NULLIF(CNT, 0) CNT FROM result order by 1
        ;