將nvarchar轉換爲在SQL Server中浮動時出錯

Question

我想在我的SQL中查看前3個月的會話小時數（hrs列聲明爲nvarchar），如果我想查看數據，則需要將數據轉換爲浮點數最近3個月，但我得到了一些錯誤

SELECT sum(convert(float, hrs)) 
FROM companysonvinunitvenue 
WHERE date >= DATEADD(day, -90, GETDATE()) 

錯誤：

Msg 8114, Level 16, State 5, Line 1
Error converting data type nvarchar to float.

SELECT sum(cast(hrs as float)) 
FROM companysonvinunitvenue 
WHERE date >= DATEADD(day, -90, GETDATE()) 

錯誤：

Msg 8114, Level 16, State 5, Line 1
Error converting data type nvarchar to float.

SELECT CAST(CAST (hrs AS NUMERIC(19,4)) AS INT) 
FROM companysonvinunitvenue 
WHERE date >= DATEADD(day, -90, GETDATE()) 

錯誤：

Msg 8114, Level 16, State 5, Line 1
Error converting data type nvarchar to numeric.

SELECT CAST(CAST (hrs AS int) AS INT) 
FROM companysonvinunitvenue 
WHERE date >= DATEADD(day, -90, GETDATE()) 

錯誤：

Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the nvarchar value '02:00:00 ' to data type int.

我想這四種方式，並在所有的人都失敗了，我需要怎麼做呢？

Answer 1

首先，對日期時間值使用date and time data types。你會避免這些和其他許多麻煩，並且還可以節省磁盤空間，RAM，減少讀取次數等等。

至於你的問題。對於SQL Server 2008+

DECLARE @companysonvinunitvenue TABLE (hrs NVARCHAR(10)) 

INSERT @companysonvinunitvenue VALUES ('02:00:00'),('21:31:03') 

SELECT DATEPART(HOUR,CAST(hrs AS time)) 
FROM @companysonvinunitvenue 

對於其他版本

DECLARE @companysonvinunitvenue TABLE (hrs NVARCHAR(10)) 

INSERT @companysonvinunitvenue VALUES ('02:00:00'),('21:31:03') 

SELECT cast(LEFT(hrs,2) AS INT) 
FROM @companysonvinunitvenue