示例：计算累计总和

SELECT dept_id, start_date, name, salary,
  SUM(salary) OVER (PARTITION BY dept_id ORDER BY
  start_date ROWS BETWEEN UNBOUNDED PRECEDING AND
  CURRENT ROW) 
FROM emp1 
ORDER BY dept_id, start_date;

DepartmentID  start_date   name    salary   sum(salary)

--------     ----------    ----    ------   -----
100          1996-01-01    Anna   	18000   	18000
100      				1997-01-01    Mike    28000    46000
100      				1998-01-01    Scott   29000    75000
100      				1998-02-01    Antonia 22000    97000
100      				1998-03-12    Adam    25000   122000
100      				1998-12-01    Amy     18000   140000
200      				1998-01-01    Jeff    18000    18000
200      				1998-01-20    Tim     29000    47000
200      				1998-02-01    Jim     22000    69000
200      				1999-01-10    Tom     28000    97000
300      				1998-03-12    Sandy   55000    55000
300      				1998-12-01    Lisa    38000    93000
300      				1999-01-10    Peter   48000   141000