将子字符串数字从 nvarchar 转换为数字

Question

I know there are a lot of question on here about converting strings to integers in SQL but I have not been able to get anything to work with a value returned by substring. I am using the substring function to return only the numbers from a string so I can compare the values but I'm getting an error.

Error converting data type nvarchar to numeric.

I have tried running the below:

select 
   sy.ResourceID,
   sy.ResourceType,
   sy.Name0,
   sy.SMS_Unique_Identifier0,
   sy.Resource_Domain_OR_Workgr0,
   sy.Client0,
   Cast(Cast (substring(bio.SMBIOSBIOSVersion0,9,6)as Numeric(4,2)) as Int) as 'BiosVersion'
from v_R_system sy 
   inner join v_GS_FIRMWARE firm on firm.ResourceID = sy.ResourceId 
   inner join v_GS_COMPUTER_SYSTEM CS on cs.ResourceID = sy.ResourceId 
   inner join v_GS_PC_BIOS Bio on bio.ResourceID = sy.ResourceId 
where firm.SecureBoot0 != 1 and cs.Model0 = 'HP EliteBook 840 G3'

Casting as int rounds to 1 , when it should be 01.43 based on what the substring gets. When I remove the cast as int and just convert as numeric it truncates it 1.42 . When I try compare it to a value I specify I still get the error stated above.

select 
   sy.ResourceID,
   sy.ResourceType,
   sy.Name0,
   sy.SMS_Unique_Identifier0,
   sy.Resource_Domain_OR_Workgr0,
   sy.Client0,
   Cast(substring(bio.SMBIOSBIOSVersion0,9,6)as Numeric(4,2)) as 'BiosVersion'
from v_R_system sy 
   inner join v_GS_FIRMWARE firm on firm.ResourceID = sy.ResourceId 
   inner join v_GS_COMPUTER_SYSTEM CS on cs.ResourceID = sy.ResourceId 
   inner join v_GS_PC_BIOS Bio on bio.ResourceID = sy.ResourceId 
where firm.SecureBoot0 != 1 and cs.Model0 = 'HP EliteBook 840 G3' 
   and Cast(substring(bio.SMBIOSBIOSVersion0,9,6)as Numeric(4,2)) < 01.42

is this an issue with my comparison actually or an issue with the way I'm formatting?

Product:

• Microsoft SQL Server Standard (64-bit)
• Version: 2016 (v13.0.4604.0)
• Language: English (United States)
• Collation: SQL_Latin1_General_CP1_CI_AS

Answer 1

简单的修复是try_cast()而不是cast() ：

try_Cast(try_Cast(substring(bio.SMBIOSBIOSVersion0, 9, 6)as Numeric(4,2)) as Int) as BiosVersion

Answer 2

You are most likely getting a leading space through your substring . Try starting at 10 instead of 9. And make use of the where clause to limit you result set to only qualifying records

cast(substring(bio.SMBIOSBIOSVersion0,10,6) as numeric(4,2))

Answer 3

As an alternate to cast, you can coerce a 'numeric context' by adding 0.

select ...
0+substring(bio.SMBIOSBIOSVersion0,9,6) as 'BiosVersion'

It may depend on your database flavor and version. Please specify.

It is possible that you have data where the specified substring is not numeric. In that case, try_cast will work (see other answer).

Answer 4

Casting as int rounds to 1 , when it should be 01.43 based on what the substring gets. When I remove the cast as int and just convert as numeric it truncates it 1.42 .

First, the rounding problem. If you have more precision than two digits, you need to cast to a numeric type that includes all the possible digits, since any of those digits might turn out to be a tie-breaker for which way to round. Once you complete that cast, use the ROUND() function to get the precision you actually need.

You also seem to care about the leading 0 , at least for the SELECT clause. I advise against that. Let client code worry about precise formatting for numbers and dates. But if you really need this, you want the FORMAT() function. Again, use this after casting to numeric, but you can use it to replace ROUND() . The import thing is this produces a string value again, rather than a numeric value, so only use it for the SELECT clause when the client program or report expects string data.

Finally, there is a situation in the WHERE clause with some invalid casts. The problem here is _you cannot force SQL Server to run conditions in the WHERE clause in any specific order. Sql Server will process those filters in the order it believes is most efficient, even if some are needed sooner for logical reasons.

With that in mind, you should look at using TRY_CAST() in the WHERE clause. This will allow you to safely cast these substring to numeric values, even before other necessary filters are complete.