USE tpch1;
select avg(if(n_regionkey>0, n_nationkey, NULL)) from nation;
avg(if(n_regionkey>0, n_nationkey, NULL))
12.5000
select r_regionkey as r, NULL as n from region union select r_regionkey, r_name from region order by 1, 2;
r	n
0	NULL
0	AFRICA
1	NULL
1	AMERICA
2	NULL
2	ASIA
3	NULL
3	EUROPE
4	NULL
4	MIDDLE EAST