An example to illustrate how unions work. This query unions the os_currentstep and os_historystep table to get the timestamps of all status transitions for issues. You'd be much better off using changegroup/changeitem without a union, but I needed an example :)

union.sql

select
  p.pkey || '-' || ji.issuenum AS issue_key
  ,cs.start_date
from jiraissue ji
join project p
  on ji.project = p.id
join os_currentstep cs
  on ji.workflow_id = cs.entry_id

union

select
  p.pkey || '-' || ji.issuenum AS issue_key
  ,hs.start_date
from jiraissue ji
join project p
  on ji.project = p.id
join os_historystep hs
  on ji.workflow_id = hs.entry_id