oracle报错——字符集不匹配

在执行查询下面语句时，出现了字符集不匹配的错误。

select 
  zsbh as 证书编号,
  zxri as 登记日期,
  '' as 需注销证书编号,
  '' as 需注销注销日期,
from cqk.dbo.zsb1
union
select 
  zsbh as 证书编号,
  zxri as 登记日期,
  '' as 需注销证书编号,
  '' as 需注销注销日期,
from cqk.dbo.zsb2

经过查资料，发现是字段类型为 NVARCHAR2() 常会发生的问题。

改为：

select 
  zsbh as 证书编号,
  zxri as 登记日期,
  N'' as 需注销证书编号,
  N'' as 需注销注销日期,
from cqk.dbo.zsb1
union
select 
  zsbh as 证书编号,
  zxri as 登记日期,
  N'' as 需注销证书编号,
  N'' as 需注销注销日期,
from cqk.dbo.zsb2