select * from @a a left join @b b on a.a=b.a and a.b=b.b and a.c=b.c
select a, b, min(c) from @a group by a,b having count(*)=1
select * from @a a where not exists(select 1 from @b where a=a.a and b=a.b and c=a.c) union all select * from @b b where not exists(select 1 from @a where a=b.a and b=b.b and c=b.c)
union all
select * from @b b where not exists(select 1 from @a where a=b.a and b=b.b and c=b.c)