Error in writting query

Question

iam having this following query in which FirstName,MiddleName,LastName columns in tables and their datatype
is nvarchar2 i am writing this query
1) SELECT NVL(Employee_Mst.FirstName, '  ' ) || ' '
*|| NVL(Employee_Mst.MiddleName, ' ') || ' '*
*|| NVL(Employee_Mst.LastName,cast(' ' as nvarchar2) ) as Doctor_Name*---- error coming in this line
from Employee_Mst
this gives me this error
Error at Command Line:3 Column:64
Error report:
SQL Error: ORA-00906: missing left parenthesis
00906. 00000 - "missing left parenthesis"
*Cause:   
*Action:
when i use this query this compiles with no problem
2) SELECT NVL(Employee_Mst.FirstName, '  ' ) || ' '
*|| NVL(Employee_Mst.MiddleName, ' ') || ' '*
*|| NVL(Employee_Mst.LastName,' ' ) as Doctor_Name------------- error is coming in thus line*
from Employee_Mst                                                              as marked by SQL Developer
but problem arise when i use this second query which compiles with no problem
with inner join with other tables there a actually so many tables with which
inner join is taking place only i have shown you a part of that query where
error is coming
error:
Error(387,13): PL/SQL: ORA-12704: character set mismatch
i am not understanding what is actually happening plz
help me

Answer 1

SELECT NVL(Employee_Mst.FirstName,' ') || ' ' ||
       NVL(Employee_Mst.MiddleName,' ') || ' ' ||
       NVL(Employee_Mst.LastName,cast(' ' as nvarchar2(1))) as Doctor_Name /* length must be specified */
  from Employee_MstCan't you just
SELECT replace(ltrim(Employee_Mst.FirstName || ' ' ||
                     Employee_Mst.MiddleName || ' ' ||
                     Employee_Mst.LastName,' '
              ) as Doctor_Name
  from Employee_MstRegards
Etbin