MySQL：多字段复合统计

Question

菜鸟求助：eves表中有三个国家字段，nat1, nat2, nat3，nat1为非空，其它两个不一定有数据。现在想统计3列中每个国家（包括出现在nat2和nat3里的国家）出现的次数，请问如何SELECT？感谢！！试过COUNT和GROUP BY，似乎无法实现啊……

Answer 1

select a.nat, count(1) as num from (

(select nat1 as nat from Test where nat1 is not null)
union all
(SELECT nat2 AS nat FROM Test WHERE nat2 IS NOT NULL)
UNION ALL
(SELECT nat3 AS nat FROM Test WHERE nat3 IS NOT NULL)

) as a group by a.nat