sql server 学习笔记 ( row_number, rank, dense_rank over partition by order by )

refer :

https://blog.csdn.net/winer2008/article/details/4283539

https://www.cnblogs.com/linJie1930906722/p/6036053.html

https://www.c-sharpcorner.com/blogs/rownumber-function-with-partition-by-clause-in-sql-server1

https://stackoverflow.com/questions/6841605/get-top-1-row-of-each-group/12190849

要想 group by get first row, 网上给出的方案多半是长这样的. 

;WITH cte AS
(
   SELECT *,
         ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) AS rn
   FROM DocumentStatusLogs
)
SELECT *
FROM cte
WHERE rn = 1

with xx as 类似 temp table 这里不多讲. 

row_number 就是做出 1,2,3 这种号码. 

而 over (partition by column order by column) 则是分组和排序. 

也就是说这个 row_number 并不是基于 当下的表,  而是基于分组排序后的表.

所以后来 where rn = 1 就能把第一条 row 取出来了. 

除了常用的 row_number

还有 rank 和 dense_rank

它们有一点区别而且. 

row_number 是 1,2,3,4 几时值相同依然会给一个新号码

rank 则是 1,2,2,4 如果值相同会公用号码, 然后会跳号, 3 就没了. 

dense_rank 则是 1,2,2,3 值相同公用号码，但是它不会跳，所以 3 依然会有.

;WITH cte AS(SELECT*,
         ROW_NUMBER()OVER(PARTITIONBY DocumentID ORDERBY DateCreated DESC)AS rn
   FROM DocumentStatusLogs
)SELECT*FROM cte
WHERE rn =1