用于返回先前匹配记录的 SQL 子查询的替代方法

Question

SQL Azure 数据库 V12 (SQL Server 2016)

给定以下基本表结构：

MyTable
==============
Id int PK
TheDate datetime2 not null
TheValue varchar(50) not null
-- other fields

还有下面的SQL：

select 
    (   select count(*) 
        from MyTable mt2 
        where 
            mt2.TheValue = mt1.TheValue
            and mt2.TheDate < mt1.TheDate
    ) as PriorCount
    , mt1.TheDate, mt1.TheValue 
from MyTable mt1
where mt1.TheDate between '2016-01-01' and '2017-01-01'
order by mt1.TheDate desc

示例输出：

PriorCount   TheDate                 TheValue
===============================================
   1         2016-06-01 00:00:00     Foo
   2         2016-05-01 00:00:00     Bar
   1         2016-04-01 00:00:00     Bar
   0         2016-03-01 00:00:00     Foo
   0         2016-02-01 00:00:00     Bar

我已经查看了OVER Clause，但找不到任何东西来返回之前的计数。是否有替代 SQL 查询来返回 PriorCount 而无需子选择？

Answer 1

您可以将COUNT 与ORDER BY 子句一起使用：

select count(*) over (partition by TheValue order by TheDate) - 1 as PriorCount,
       mt1.TheDate, mt1.TheValue 
from MyTable mt1
where mt1.TheDate between '2016-01-01' and '2017-01-01'
order by mt1.TheDate desc

编辑：

如果要将COUNT 应用于整个表，则可以使用以下查询：

select PriorCount, TheDate, TheValue
from (
   select count(*) over (partition by TheValue 
                         order by TheDate) - 1 as PriorCount,
          TheDate, TheValue 
   from MyTable 
   order by TheDate desc) as t
where t.TheDate between '2016-01-01' and '2017-01-01'