Question

在SQL Server中，我有一个整数，日期和字符串列表。例如，

number  datetime             string
6       2011-09-22 12:34:56  nameOne
6       2011-09-22 1:23:45   nameOne
6       2011-09-22 2:34:56   nameOne
5       2011-09-22 3:45:01   nameOne
5       2011-09-22 4:56:01   nameOne
5       2011-09-22 5:01:23   nameOne
7       2011-09-21 12:34:56  nameTwo
7       2011-09-21 1:23:45   nameTwo
7       2011-09-21 2:34:56   nameTwo
4       2011-09-21 3:45:01   nameTwo
4       2011-09-21 4:56:01   nameTwo
4       2011-09-21 5:01:23   nameTwo

我想写一个SQL语句，只输出那些数字是每个字符串最大值的行。在这个例子中，

number  datetime             string
6       2011-09-22 12:34:56  nameOne
6       2011-09-22 1:23:45   nameOne
6       2011-09-22 2:34:56   nameOne
7       2011-09-21 12:34:56  nameTwo
7       2011-09-21 1:23:45   nameTwo
7       2011-09-21 2:34:56   nameTwo

我知道我可以遍历字符串列中的每个字符串，然后获取该字符串的最大值，然后选择与该字符串匹配的行。例如，

declare @max int
declare my_cursor cursor fast_forward for
    select distinct string
    from table
open my_cursor
fetch next from my_cursor into @string
while @@fetch_status = 0
begin
    set @max = (select max(number) from table where string = @string)
    select * from table where number = @max
    fetch next from my_cursor into @string
end
close my_cursor
deallocate my_cursor

但是，我想知道是否有办法完成此任务 WITHOUT 使用循环（例如，通过使用聚合函数和分组）。

Answer 1

;WITH T as
(
SELECT *,
       RANK() OVER (PARTITION BY string ORDER BY number DESC) RN
FROM YourTable
)
SELECT number,
       datetime,
       string
FROM T
WHERE RN=1;

Answer 2

WITH maxes AS (
    SELECT string, MAX(number) AS max_number
    FROM tbl
    GROUP BY string
)
SELECT tbl.*
FROM tbl
INNER JOIN maxes
    ON maxes.string = tbl.string AND maxes.max_number = tbl.number

如何在SQL中选择每列最大值？

2 个答案: