sql：从两个表中排序并按日期排序

Question

I met a problem in my iPhone App - I create tow tables with sqlite3:

create table A (Name varchar(50), Added datetime);
create table B (UserID varchar(50), Username varchar(50), Created datetime);

I need to get all the values of the two tables ordered by time, which is like:

Alen  2011-06-25 17:56:00
12    Fire      2011-06-26 17:56:00
Bale  2011-07-01 17:56:00

As you see there is no relationship between of the tables, I've no idea about it.

The app is undergoing, and it's difficult to redesign the DB. I'd like to know the solution based on the current DB schema (this is also the boss's requirement).

Answer 1

SELECT NULL AS Col1, Name AS Col2, Added AS Col3
FROM A
UNION ALL 
SELECT UserID AS Col1, Username AS Col2, Created AS Col3
FROM B
ORDER BY 3