(Borrowed from Connor McDonald's Twitter account.)
SQL> select
2 sum((
3 4 / (8*(level-1)+1) -
4 2 / (8*(level-1)+4) -
5 1 / (8*(level-1)+5) -
6 1 / (8*(level-1)+6)
7 ) / power(16,level-1)) pi
8 from dual connect by level <= 10;
PI
----------
3.14159265