I have a class, called A, with one-to-many parent-child relationship with its child, called B. I need to retrieve a list of A met specific conditions. Not all attributes of A need to be displayed in the list, nor the attributes of the B class, but some statistic data of B class such as the count, the average of one attribute value, etc. With the transfer data object approach, I can have two approaches with the A's children class B: fetch all its children B and write some methods of the TDO to get those statistic data, or apply the HQL aggregate functions in the HQL.

Which approach is better?

Thanks for your inputs.

use HQL, you can use the databases algorithms for the aggregates (which are ways more effective that iterating back and forth in your code)

Thanks for your input, Mike.

The calculation the statistis data is not necessary using iterators to walk over the data, at least from the API. The count, for example, can be obtained by size method of the collection class.

of course but a database will always be more efficient at these kinds of things. in addition to that you are able to reduce the memory footprint of your app since you dont have to load all the objects into memory.