There are some questions your interviewer has no business asking, and this is one.  But while you may feel like answering, "none of your business, "  naturally you can't. Some interviewers ask this question on the chance you admit to something, but if not, at least they'll see how you think on your  feet. 

Some unprepared candidates, flustered by this question,  unburden themselves of guilt from their personal life or career, perhaps expressing regrets regarding a parent, spouse, child, etc. All such answers can be disastrous.

BEST ANSWER:  As with faults and weaknesses,  never confess a regret.  But don't seem as if you're stonewalling either.

Best strategy:  Say you harbor no regrets, then add a principle or habit you practice regularly for healthy human relations.

Example:  Pause for reflection, as if the question never occurred to you.   Then say, "You know, I really can't think of anything."  (Pause again, then add): "I would add that as a general management principle, I've found that the best way to avoid regrets is to avoid causing them in the first place.  I practice one habit that helps me a great deal in this regard.  At the end of each day, I mentally review the day's events and conversations to take a second look at the people and developments I'm involved with and do a doublecheck of what they're likely to be feeling.  Sometimes I'll see things that do need more follow-up, whether a pat on the back, or maybe a five minute chat in someone's office to make sure we're clear on things.whatever."

• Presentation Layer
• Business Logic Layer
• Data Access Layer

In most of the cases the data entered by the end user needs some kind of validation or further processing. This is the responsibility of Business Logic Layer.

Finally, your application data needs to be stored and retrieved in some data store (RDBMS, XML etc.). This task is handled by Data Access Layer.

In short, the process works like this:

• User requests for some application data.
• The data access layer retrieves the data and is forwarded to the presentation layer via business logic layer. Sometimes data access layer gives this data directly to presentation layer.
• Presentation layer receives the data to be displayed via business logic layer.
• The user changes the data and initiates the appropriate action (such as insert, or update).
• The business logic layer validates the data submitted by the user.
• If the data is valid it is handed over to data access layer for updating into the database.

• The applications gets divided in logically isolated pieces reducing tight coupling between the UI, business processes and database.
• Change in the underlying database and data access methods do not have any effect on the presentation layer or client application.
• Client application no longer has SQL statements embedded in it. This makes it de-coupled from rest of the application.
• Table and column names can be effectively eliminated from the client-side code.
• The client application is unaware from where data comes (location transparency).
• It becomes easier to modify or extend your application, without breaking or recompiling the client-side code.