在查询中使用TOP 1的问题 [英] Problem with the use of TOP 1 in a query

问题描述

我写了以下查询来获取日期，删除它的时间部分并添加我想要的时间. 如果我在不使用TOP子句的情况下运行此查询，则效果很好.但是，当我添加它时，它返回以下异常:从字符串转换日期和/或时间时转换失败."

I wrote the following query to obtain a date, remove it's time part and add the time I wanted. If I run this query without the TOP clause, it works well. But when I add it, it returns the following exception: "Conversion failed when converting date and/or time from character string."

以下是查询:

SELECT TOP 1 
CONVERT(DateTime, (CONVERT(varchar(50),CONVERT(Date, VRSAS.EventOn)) 
+ ' ' + 
CONVERT(varchar(50), CONVERT(Time, '23:30')))) E 
FROM ViewRangeSheetActualStatus VRSAS 
Where VRSAS.EventOn <= '2010-07-31' 
AND VRSAS.[Status] = 1
 order by VRSAS.RangeSheet

EventOn字段的类型为DateTime.

The field EventOn is of type DateTime.

可能会发生什么?

推荐答案

为此目的，我已经很容易地重现了.我发现使用DATEADD解决了该问题

I've reproduced quite easily this end. I found using DATEADD resolved it

DATEADD(MINUTE, 23*60 + 30, CONVERT(DATETIME,CONVERT(DATE, VRSAS.EventOn)))

但是我实际上还不确定为什么.重现步骤如下.

But I'm not actually sure why yet. Steps to reproduce below.

CREATE TABLE ViewRangeSheetActualStatus
(EventOn DATETIME,
[Status] BIT,
RangeSheet INT
)

INSERT INTO [dbo].[ViewRangeSheetActualStatus]([EventOn], [Status], [RangeSheet])
SELECT '20100903 11:02:39.517', 1, 1 UNION ALL
SELECT '20100731 11:03:23.577', 1, 1 UNION ALL
SELECT '20100731 00:00:00.000', 1, 1

/*Selects ALL records - No error*/
SELECT 
CONVERT(DateTime, (CONVERT(varchar(50),CONVERT(Date, VRSAS.EventOn)) 
+ ' ' + 
CONVERT(varchar(50), CONVERT(Time, '23:30')))) E 
FROM ViewRangeSheetActualStatus VRSAS 

/*Selects top (1) record - Error!*/   
SELECT top (1)
CONVERT(DateTime, (CONVERT(varchar(50),CONVERT(Date, VRSAS.EventOn)) 
+ ' ' + 
CONVERT(varchar(50), CONVERT(Time, '23:30')))) E 
FROM ViewRangeSheetActualStatus VRSAS 

看看执行计划中的ComputeScalar属性，两者是不同的.

Looking at the ComputeScalar properties in the execution plan the two are different.

全部

(CONVERT(datetime,(CONVERT(varchar(50),CONVERT(date,[ViewRangeSheetActualStatus].
 [EventOn] as [VRSAS].[EventOn],0),0)+[@1])+CONVERT(varchar(50),CONVERT(time(7), 
 [@2],0),0),0))

前1名

(CONVERT(datetime,(CONVERT(varchar(50),CONVERT(date,[ViewRangeSheetActualStatus].
 [EventOn] as [VRSAS].[EventOn],0),121)+' ')+'23:30:00.0000000',0))

在最终转换为datetime之前，第一个生成包含以下内容的varchar

Before the final conversion to datetime the first one produces a varchar containing the following

------------------------------
Sep  3 2010 11:30PM
Jul 31 2010 11:30PM
Jul 31 2010 11:30PM

第二个版本会生成一个包含varchar的

The second version produces a varchar containing

------------------------------
2010-09-03 23:30:00.0000000

是.0000000导致问题回退到datetime的原因.我不知道为什么在查询中添加TOP会导致这种完全不相关的行为更改.

It is the .0000000 that causes the problem casting back to datetime. I have no idea why the addition of TOP to the query would cause this completely unrelated change in behaviour.

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