现在是这种情况 通过查询得到两个结果集A、B 其中A、B中都有companyid和productid字段
现在我要实现A和B通过这两个字段联合 也就是join 如果A中有的B中不存在，或者B中有的在A中不存在 都要取出来，不存在的就用空值。现在不知道该怎么合并，求大牛指导
------解决方案--------------------
select a.*,b.*
from a fulljoin b on a.companyid=b.companyid and a.productid=b.productid
------解决方案--------------------
select * from A left join B on A.companyid=B.companyid and A.productid=B.productid
union
select * from B left join A on A.companyid=B.companyid and A.productid=B.productid
------解决方案--------------------
select a.companyid as a_companyid, a.productid as a_productid, b.companyid as b_companyid, b.productid as b_productid
from a left join b on a.companyid = b.companyid and a.productid = b.productid
where b.companyid is null
union all
select a.companyid as a_companyid, a.productid as a_productid, b.companyid as b_companyid, b.productid as b_productid
from b left join a on a.companyid = b.companyid and a.productid = b.productid
where a.companyid is null