using subquery instead list name an seer transform statement

i need an transform matter used churned tables establish rows update, given oracle, churned tables aren't allowed. following query relapse "ora-00971: blank set keyword" error

update
  table1 a,
  table2 b
set
  a.col1 = 'value'
where
  a.fk = b.pk
  b.col2 ('set values')

looking adult transform matter syntax oracle, i found following , shows subquery place list name.

when i attempted query this, i got "ora-01779: can't cgange buttress maps non key-preserved table"

update
  (
    select
      a.col1
    from
      table1 a,
      table2 b
    where
      a.fk = b.pk
      b.col2 ('set values')
  ) update_tbl
set
  update_tbl.col1 = 'value'

i rewrite query (show below) controlling an exists matter instead works fine, nonetheless still know done.

update
  table1 update_tbl
set
  update_tbl.col1 = 'value'
where
  exists (
    select
      1
    from
      table1 a
      table2 b
    where
      a.fk = b.pk
      b.col2 ('set values')
      update_tbl.pk = a.pk
  )

thanks!
-nate