MSSQL 在 GROUP BY 中检索值 1 次

Question

我有一个 table 包含以下数据：

CREATE TABLE [dbo].[Products](
[ProductNR] [varchar](14) NULL,
[Location] [int] NULL,
[Date] [datetime] NULL
);

 INSERT INTO Products (ProductNR, Location, Date)
 VALUES 
 ('1234' ,1, '2016-01-17 12:30:50.010'),
 ('4567' ,1, '2016-03-17 12:30:50.010'),
 ('8978' ,1, '2016-04-17 12:30:50.010'),
 ('2578' ,1, '2016-05-17 12:30:50.010'),
 ('1234' ,2, '2016-06-17 12:30:50.010'),
 ('1234' ,3, '2016-07-17 12:30:50.010');

select count (distinct ProductNR)
from Products

结果：

|Count|
|  4  |

但是当我像下面这样使用 GROUP BY 语句时：

select count (distinct ProductNR) AS Count
from Products
group by MONTH(date)

结果：

|Count|
| 1   |
| 1   |
| 1   |
| 1   |
| 1   | 
| 1   |

总共 6，但我想检索的是整个 table/GROUP BY 语句中的 ID，这意味着我只想返回 4 行回到计算每个 ID 的第一个注册日期的地方。

期望的结果，其中 ProductNR 1234 仅检索一次：

|Count|
| 1   |
| 1   |
| 1   |
| 1   |

Answer 1

首先获取每个 ProductNR 的最短日期，然后按月分组

select  count (distinct ProductNR) AS Count
from
(
    select  ProductNR, Date = min(Date)
    from    Products
    group by ProductNR
) d
group by MONTH(Date)

Answer 2

我想你想要达到的是这个

select count (distinct ProductNR) AS Count,min(date)
from Products
group by ProductNR

Answer 3

您可以添加子 select 并仅使用日期 = 产品最小日期的行

  select count (distinct ProductNR) AS Count
    from Products as Products
    where date =(select  min(date) 
                   from Products as MinProd 
                  where MinProd.ProductNR =Products.ProductNR 
                 ) 
    group by MONTH(date)

这只有在没有相同的最短日期时才有效 beater 将在子查询中使用标识列和 select 前 1（此 return 始终只有一行）

CREATE TABLE [dbo].[Products2](
[PRD_ID] int identity,
[ProductNR] [varchar](14) NULL,
[Location] [int] NULL,
[Date] [datetime] NULL
);

 INSERT INTO Products2 (ProductNR, Location, Date)
 VALUES 
 ('1234' ,1, '2016-01-17 12:30:50.010'),
 ('4567' ,1, '2016-03-17 12:30:50.010'),
 ('8978' ,1, '2016-04-17 12:30:50.010'),
 ('2578' ,1, '2016-05-17 12:30:50.010'),
 ('1234' ,2, '2016-06-17 12:30:50.010'),
 ('1234' ,3, '2016-07-17 12:30:50.010');

select count (distinct ProductNR) AS Count
from Products as Products
where date =(select  min(date) 
               from Products as MinProd 
              where MinProd.ProductNR =Products.ProductNR 
             ) 
group by MONTH(date)

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