T-SQL 获取最后的日期时间记录

Question

我的桌子是这样的：

+---------+------------------------+-------+---------+---------+
|channel  |date                    |code   |comment  |order_id |
+---------+------------------------+-------+---------+---------+
|1        |2017-10-27 12:04:45.397 |2      |comm1    |1        |
|1        |2017-10-27 12:14:20.997 |1      |comm2    |1        |
|2        |2017-10-27 12:20:59.407 |3      |comm3    |1        |
|2        |2017-10-27 13:14:20.997 |1      |comm4    |1        |
|3        |2017-10-27 12:20:59.407 |2      |comm5    |1        |
|3        |2017-10-27 14:20:59.407 |1      |comm6    |1        |
+---------+------------------------+-------+---------+---------+

我希望得到这样的结果：

+---------+------------------------+-------+---------+
|channel  |date                    |code   |comment  |
+---------+------------------------+-------+---------+
|1        |2017-10-27 12:14:20.997 |1      |comm2    |
|2        |2017-10-27 13:14:20.997 |1      |comm4    |
|3        |2017-10-27 14:20:59.407 |1      |comm6    |
+---------+------------------------+-------+---------+

每个频道总是有 1 条记录，order_id = x 和最大日期。通道总数是恒定的。 我的查询有效，但随着表的增长，我担心性能。执行三个几乎相同的查询似乎并不聪明。

select
    *
from
    (select top(1)
        channel,
        date,
        code,
        comment
    from
        status
    where
        channel = 1 and
        order_id = 1 and
        cast(date as date) = '2017-10-27'
    order by 
        date desc) channel1
union
select 
    *
from
    (select top(1)
        channel,
        date,
        code,
        comment
    from
        status
    where
        channel = 2 and
        order_id = 1 and
        cast(date as date) = '2017-10-27'
    order by 
        date desc) channel2
union
select 
    *
from
    (select top(1)
        channel,
        date,
        code,
        comment
    from
        status
    where
        channel = 3 and
        order_id = 1 and
        cast(date as date) = '2017-10-27'
    order by 
        date desc) channel3

我该如何改进？

Answer 1

另一个选项是使用 WITH TIES 子句。没有子查询或额外的字段。

Select top 1 with ties *
 From  YourTable
 Order By Row_Number() over (Partition By channel order by date desc)

Answer 2

尝试使用ROW_NUMBER() 函数和派生表。它将为您省去很多麻烦。试试：

select channel
       ,date
       ,code
       ,comment
from
(select *
       ,row_number() over(partition by channel order by code asc) rn --probably don't need asc since it is ascending by default
from mytable) t
where t.rn = 1

Answer 3

假设您想要每个频道的最新行，这将起作用。

SELECT *
FROM (
    SELECT
        ROW_NUMBER() OVER (PARTITION BY s.channel ORDER BY [date] DESC) AS rn,
        *
    FROM [status] AS s
) AS t
WHERE t.rn = 1