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ROW_NUMBER

Syntax

Description of the illustration row_number.gif

Purpose

ROW_NUMBER is an analytic function. It assigns a unique number to each row to which it is applied (either each row in the partition or each row returned by the query), in the ordered sequence of rows specified in the order_by_clause, beginning with 1.

By nesting a subquery using ROW_NUMBER inside a query that retrieves the ROW_NUMBER values for a specified range, you can find a precise subset of rows from the results of the inner query. This use of the function lets you implement top-N, bottom-N, and inner-N reporting. For consistent results, the query must ensure a deterministic sort order.

You cannot nest analytic functions by using ROW_NUMBER or any other analytic function for expr. However, you can use other built-in function expressions for expr. Refer to "About SQL Expressions" for information on valid forms of expr.

Examples

The following example finds the three highest paid employees in each department in the hr.employees table. Fewer than three rows are returned for departments with fewer than three employees.

SELECT department_id, first_name, last_name, salary
FROM
(SELECTdepartment_id, first_name, last_name, salary,ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary desc) rnFROM employees
)
WHERE rn <= 3
ORDER BY department_id, salary DESC, last_name;

The following example is a join query on the sh.sales table. It finds the sales amounts in 2000 of the five top-selling products in 1999 and compares the difference between 2000 and 1999. The ten top-selling products are calculated within each distribution channel.

SELECT sales_2000.channel_desc, sales_2000.prod_name,sales_2000.amt amt_2000,  top_5_prods_1999_year.amt amt_1999,sales_2000.amt  - top_5_prods_1999_year.amt amt_diff
FROM
/* The first subquery finds the 5 top-selling products per channel in year 1999. */(SELECT channel_desc, prod_name, amtFROM(SELECT channel_desc, prod_name, sum(amount_sold) amt,ROW_NUMBER () OVER (PARTITION BY channel_descORDER BY SUM(amount_sold) DESC) rnFROM sales, times, channels, productsWHERE sales.time_id = times.time_idAND times.calendar_year = 1999AND channels.channel_id = sales.channel_idAND products.prod_id = sales.prod_idGROUP BY channel_desc, prod_name)WHERE rn <= 5) top_5_prods_1999_year,
/* The next subquery finds sales per product and per channel in 2000. */(SELECT channel_desc, prod_name, sum(amount_sold) amtFROM sales, times, channels, productsWHERE sales.time_id = times.time_idAND times.calendar_year = 2000AND channels.channel_id = sales.channel_idAND products.prod_id = sales.prod_idGROUP BY channel_desc, prod_name) sales_2000
WHERE sales_2000.channel_desc = top_5_prods_1999_year.channel_descAND sales_2000.prod_name = top_5_prods_1999_year.prod_name
ORDER BY sales_2000.channel_desc, sales_2000.prod_name
;
CHANNEL_DESC    PROD_NAME                                          AMT_2000   AMT_1999   AMT_DIFF
--------------- --------------==-------------------------------- ---------- ---------- ----------
Direct Sales     17" LCD w/built-in HDTV Tuner                     628855.7 1163645.78 -534790.08
Direct Sales     Envoy 256MB - 40GB                               502938.54  843377.88 -340439.34
Direct Sales     Envoy Ambassador                                2259566.96 1770349.25  489217.71
Direct Sales     Home Theatre Package with DVD-Audio/Video Play  1235674.15 1260791.44  -25117.29
Direct Sales     Mini DV Camcorder with 3.5" Swivel LCD           775851.87 1326302.51 -550450.64
Internet         17" LCD w/built-in HDTV Tuner                     31707.48   160974.7 -129267.22
Internet         8.3 Minitower Speaker                            404090.32  155235.25  248855.07
Internet         Envoy 256MB - 40GB                                28293.87  154072.02 -125778.15
Internet         Home Theatre Package with DVD-Audio/Video Play   155405.54  153175.04     2230.5
Internet         Mini DV Camcorder with 3.5" Swivel LCD            39726.23  189921.97 -150195.74
Partners         17" LCD w/built-in HDTV Tuner                    269973.97  325504.75  -55530.78
Partners         Envoy Ambassador                                1213063.59  614857.93  598205.66
Partners         Home Theatre Package with DVD-Audio/Video Play   700266.58  520166.26  180100.32
Partners         Mini DV Camcorder with 3.5" Swivel LCD           404265.85  520544.11 -116278.26
Partners         Unix/Windows 1-user pack                         374002.51  340123.02   33879.4915 rows selected.

2. 我们可以用它来进行去重复

select * from (

select *, row_number() over( partition by cloumn1,column2 order by column3,column4) rn

) x where x.rn=1;

需要注意：

在同一组内部，如果order by 后面的排序列值相同的话，在逻辑意义上他们就是重复的，虽然它们最后得到的rn 不一样，因为row_number必须唯一.

但这个时候单独执行下面语句时，某行记录被分配到的rn=1, 而执行整体的时候这行记录并不一定会被分配rn=1.

select *, row_number() over( partition by cloumn1,column2 order by column3,column4) rn