with tmp as (
select 1 as a, 2 as b
union all
select 1 as a, 2 as b
union all
select 1 as a, null as b
)
select * from tmp;
with tmp as (
select 1 as a, 2 as b
union all
select 1 as a, 2 as b
union all
select 1 as a, null as b
)
select count(*), count(b) from tmp;
各列在行上的并集