Mapping with SQL

Question

Hello I have one general question.
At the moment everything looks like this:
database table:
name | firstdate | seconddate
asdf | 06/02/06 | 06/02/18
java object:
name | firstdate | seconddate
asdf | 06/02/06 | 06/02/18
I try to achieve adding a column to the java object in this way:
java object:
name | firstdate | seconddate | duration
asdf | 06/02/06 | 06/02/18 | 12
The values should be queried from the table like
SELECT name, firstdate, seconddate, (seconddate-firstdate) duration FROM table
Does anyone knows if that is possible, and when, is there an example how to achieve that?
Thanks for replying.

Answer 1

If the value is just a computed value, you may be better off computing it in Java, instead of the database. You would not even need a variable, just have a getDuration method that computed the duration from the two dates.
If you really wanted the variable you could lazy initialize it, and reset it when either of the dates are changed.
If you must compute the value in the database you could either define a view in the database that adds the column and map to the view instead of the table, or use custom SQL for all queries for the class that add in the computed column. You could also just add the column to the database.