What is the purpose of the Oracle LISTAGG function?

The `LISTAGG` function in Oracle database is an aggregate function used to concatenate multiple rows of data into a single string, separated by a specified delimiter.

In specific terms, the `LISTAGG` function takes two parameters: the columns to be concatenated and the separator. It concatenates the values of the specified columns with the specified separator and returns a string containing all the concatenated values. If no separator is specified, it defaults to using a comma as the separator.

Here is an example of using the `LISTAGG` function:

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

In the example above, we are grouping employee names from the `employees` table based on `department_id` and using the `LISTAGG` function to concatenate the employee names in each department into a single string with a comma as the separator. The final query result will include the department ID and the corresponding list of employee names.

In summary, the `LISTAGG` function allows us to merge multiple rows of data into a single string in a concise manner, and add separators when needed. This is very useful for generating reports, displaying related values, and other scenarios.