i have to display multivalued attributes into a single cell based on 'id'
SQL Developer is going to display whatever the cell value is to if you want to display what you consider to be multiple data items you need to convert them to a single data item and then they will display in a single cell.
So you need to write a query to convert the rows to columns; a topic that comes up so often it has its own FAQ.
Since this is not a SQL Developer question it should be posted in the SQL and PL/SQL forum SQL and PL/SQL
Please mark this question ANSWERED and repost it in the other forum.
Before you post there read the FAQ #4: How do I convert rows to columns. SQL and PL/SQL FAQ
See the preceding link to see examples of doing this...
Here's an example...
KEEP (DENSE_RANK LAST ORDER BY curr),',') AS employees
FROM (SELECT deptno,
ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) AS curr,
ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) -1 AS prev
GROUP BY deptno
CONNECT BY prev = PRIOR curr AND deptno = PRIOR deptno
START WITH curr = 1;