USE tpch1;
select max(K), count(*) from (select n_nationkey K from nation limit 1) a;
max(K)	count(*)
0	1
select n_nationkey K from nation limit 1;
K
0
(select n_nationkey K from nation limit 1) union (select n_nationkey K from nation limit 2) order by 1;
K
0
1
(select n_nationkey K from nation limit 1) union all (select n_nationkey K from nation limit 1);
K
0
0
(select n_nationkey K from nation limit 1) union all (select n_nationkey K from nation limit 2) limit 1;
K
0