oracle當多表union時遇到nvarchar2類型時報錯 字符集不匹配
對使用nvarchar的地方，加上 to_char( nvarchar 的變量或字段 )
如：
select to_char(name),price from aa
union all
select to_char(name),price from bb
3張表aa,bb,cc都有 name price 字段 查詢價格最高的前3位姓名
select * from(select to_char(name),price from aa
union all
select to_char(name),price from bb
union all
select to_char(name),price from cc order by price desc)where rownum<=3