i tried the following
select distinct a.EMPNBR AS "Emp Number", a.EMPNAM AS "Emp. Name",SUBSTR(b.VSTDTE,1,9) AS "Visite",(SELECT CASE when CLINTYP = '1' then 'Gen. Clinic' when CLINTYP = '3' then 'both Clinic'else 'Dent. Clinic' end FROM CLNCVST where VSTDTE='15-May-13')
from MAINFRAME.EMPBAS a inner join CLINIC.CLNCVST b ON a.EMPNBR=b.EMPNBR where b.EMPNBR IN (select EMPNBR from CLINIC.CLNCVST where VSTDTE='20-May-13')and VSTDTE= '20-May-13'
it return the value null with colum name "(SELECT CASE when CLINTYP = '1' then 'Gen. Clinic' when CLINTYP = '3' then 'both Clinic'else 'Dent. Clinic' end FROM CLNCVST where VSTDTE='15-May-13')" it consider this select statment as column header