OVER子句指定了分析函数操作的分区、排序和窗口.
Example #1: calculate a moving average
AVG(amt) OVER (ORDER BY date ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
date amt avg_amt
===== ==== =======
1-Jan 10.0 10.5
2-Jan 11.0 17.0
3-Jan 30.0 17.0
4-Jan 10.0 18.0
5-Jan 14.0 12.0
它通过一个移动窗口(3行宽)在行上运行，按日期排序.
Example #2: calculate a running balance
SUM(amt) OVER (ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
date amt sum_amt
===== ==== =======
1-Jan 10.0 10.0
2-Jan 11.0 21.0
3-Jan 30.0 51.0
4-Jan 10.0 61.0
5-Jan 14.0 75.0
它在包含当前行和所有先前行的窗口上运行.
注:对于使用OVER子句指定排序ORDER的聚合，默认窗口为UNBOUNDED PRECEDING到CURRENT ROW，因此上述表达式可以简化为，结果相同:
SUM(amt) OVER (ORDER BY date)
Example #3: calculate the maximum within each group
MAX(amt) OVER (PARTITION BY dept)
dept amt max_amt
==== ==== =======
ACCT 5.0 7.0
ACCT 7.0 7.0
ACCT 6.0 7.0
MRKT 10.0 11.0
MRKT 11.0 11.0
SLES 2.0 2.0
它在包含特定部门所有行的窗口上运行.
SQL Fiddle:http://sqlfiddle.com/#!4/9eecb7d/122