SQL & PL/SQL

I have two tables like empday1,empday2

i need to find out the empday2 columns that are not existed in empday1 table.for this purpose i am using below query like:

select count(*) from(
select * from empday2
minus
select * from empday1)

but it showing error like below:
ORA-12704: character set mismatch

when i change query in like count(*) instead of * i am getting o/p as 0.that as of my understand all the columns in empday2 are in empday1.

but why it is showing character set error when i am using *.


please help me....


Cheers.....