Some of Oracle's analytic functions allow for a windowing clause to specify a subset of the current partition, using keywords like "unbounded preceding/following", "current row", or "value_expr preceding/following" where value_expr is a physical or logical offset from the current row or value (depending on whether you have specified ROW or RANGE, respectively).
Here is an example using scott/tiger that displays employees in dept 30, and a count of the number of employees in their dept hired before them (including themselves):
select deptno,
empno,
hiredate,
count(*) over (partition by deptno
order by hiredate
range between unbounded preceding and current row) cnt_hired_before1,
count(*) over (partition by deptno
order by hiredate
range between unbounded preceding and 0 preceding) cnt_hired_before2
from emp
where deptno = 30
order by deptno, hiredate;
...can anyone provide an example or documentation where "current row" is different than "0 preceding/following"? It just seems like syntactic sugar to me...
It doesn't really matter which you use. They are two different ways of expressing the windowing, but the optimizer will perform the query the same way. The term "current row" is one that is common to multiple databases with analytic functions, not just Oracle. It's more of a stylistic difference, in the same way that some people prefer count(*) over count(1).