I need to return a query result from a datetime field in a child table that either returns NULL if "any" record that matches the condition is NULL or return MAX() if all matching records have non-null values.

To give you an example, if a person is given list of 10 tasks with a separate deadline date, I need a query that returns NULL date if any of the 10 task is not completed OR MAX() date if all tasks have been completed.

Anyone have a simple query statement for this? Thanks.