OVER clause in Oracle

The OVER clause specifies the partitioning, ordering and window "over which" the analytic function operates.

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

It operates over a moving window (3 rows wide) over the rows, ordered by date.

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

It operates over a window that includes the current row and all prior rows.

Note: for an aggregate with an OVER clause specifying a sort ORDER, the default window is UNBOUNDED PRECEDING to CURRENT ROW, so the above expression may be simplified to, with the same result:

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

It operates over a window that includes all rows for a particular dept.

SQL Fiddle: http://sqlfiddle.com/#!4/9eecb7d/122

You can use it to transform some aggregate functions into analytic:

SELECT  MAX(date)
FROM    mytable

will return 1 row with a single maximum,

SELECT  MAX(date) OVER (ORDER BY id)
FROM    mytable

will return all rows with a running maximum.

It's part of the Oracle analytic functions.