相当于A的独有+B的独有
select * from a full outer join b on a.key = b.key where a.key is null or b.key is null
此写法只在oracle中适用,在mysql中得用union
select * from a left join b on a.key = b.key where b.key is null
union
select * from a right join b on a.key = b.key where a.key is null