insert into c values ( column1, column2 ) select a.column1,a.column2 ... from a where not exists (select 1 from b where b.column1 = a.column1 ...) union
select (b.counmn1,b.column2 ...) from b where not exists (select 1 from a where b.column1 = a.column1 ...)
其中 exist where b.column1 = a.column1 ... 是所有列