Adding distinct data to jComboBox from database

I have a jComboBox which i want to fill up with the departments of the students in a database. Now the same department occurs many times in the table so i want each department name to go only once to the list of items. The present code i wrote is not giving the desired result. It puts the same department name multiple times on the ComboBox list. How can i solve this? My code to fetch department names is given below:

 conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/mydaatabase1","root","Password123");
            String sql1 = "select distinct (dept) from droptest";
            PreparedStatement pss = conn.prepareStatement(sql1);
            ResultSet rs = pss.executeQuery(sql1);
            while(rs.next())
            {
                String d = rs.getString("dept");
                jComboBox1.addItem(d);
            }

Answer

I guess, you need use group by in select data…

select columnName from tablename Group by columnName

    Select dept 
    From droptest
    Group by dept 

Group by is like distinct