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