update a set x = (
select max(y) from (
select max(d) as y from b
union all select max(e) as y from b
union all select max(f) as y from b
) as t
)

------解决方案--------------------

SQL code

update a set x = (
select max(y) from (
select d as y from b where id=a.id
union all select e as y from b  where id=a.id
union all select f as y from b  where id=a.id
) as t
)