listagg within group

`LISTAGG` is an aggregate function in SQL that concatenates the values of a specified column into a single string. The `WITHIN GROUP` clause allows you to specify the order in which the values should be concatenated.

Here's an example usage of `LISTAGG` with `WITHIN GROUP`:

SELECT department_id, 
       LISTAGG(last_name, ', ') WITHIN GROUP (ORDER BY last_name) AS employees
FROM employees
GROUP BY department_id;

In this example, the `LISTAGG` function concatenates the `last_name` values from the `employees` table for each department, with the names ordered alphabetically. The result is a comma-separated string of employees' last names for each department.