Error converting datatype nvarchar to numeric

Question

My Table Data

Amount1	Amount2	Expected Result
100	200	4.17%
A	500
500	B
20	100	1.67%

CREATE TABLE [dbo].[tblData](
    [Amount1] [nvarchar](50) NULL,
    [Amount2] [nvarchar](50) NULL
) ON [PRIMARY]

INSERT [dbo].[tblData] ([Amount1], [Amount2]) VALUES (N'100', N'200')
INSERT [dbo].[tblData] ([Amount1], [Amount2]) VALUES (N'A', N'500')
INSERT [dbo].[tblData] ([Amount1], [Amount2]) VALUES (N'', N'')
INSERT [dbo].[tblData] ([Amount1], [Amount2]) VALUES (N'500', N'B')
INSERT [dbo].[tblData] ([Amount1], [Amount2]) VALUES (N'20', N'100')

This works fine as expected when I run it on one line

SELECT TOP 1
    Concat(CAST((cast(Amount1 AS decimal(18,4))/(cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') AS Result
FROM tblData 

This fails when I run it on whole table

SELECT 
    Concat(CAST((cast(Amount1 AS decimal(18,4))/(cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') AS Result
FROM tblData

Returning the below error message.

Error converting datatype nvarchar to numeric

Answer 1

Check if columns are numeric before you do anything:

SELECT Amount1
       , Amount2
       , case when isnumeric(Amount1) = 1 and isnumeric(Amount2) = 1 then
Concat(CAST((cast(Amount1 AS decimal(18,4))/(cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') 
         else 
null
         end AS "Expected Result"
FROM tblData 

Here is a demo

If you have 0's in your data then something like this:

SELECT Amount1, Amount2, 
case when isnumeric(Amount1) = 1 and isnumeric(Amount2) = 1 and Amount2 > 0 and Amount1 > 0 then
Concat(CAST((cast(Amount1 AS decimal(18,4))/(cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') 
when isnumeric(Amount1) = 1 and isnumeric(Amount2) = 1  and Amount1 = 0 and Amount1 > 0 then
Concat(CAST((cast(1 AS decimal(18,4))/(cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') 
when isnumeric(Amount1) = 1 and isnumeric(Amount2) = 1  and Amount1 > 0 and Amount1 = 0 then
Concat(CAST((cast(Amount1 AS decimal(18,4))/(cast(1 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%') 
else 
null
end AS "Expected Result"
FROM tblData 

Here is a second demo

Answer 2

If you've got a text field that's mostly numeric but you can't rely on then instead of

cast(Amount1 AS decimal(18,4))

you could use

try_cast(Amount1 AS decimal(18,4))

Which returns Null if the conversion can't be done

I use try_convert but that's just personal preference, they're effectively the same

try_convert(decimal(18,4), Amount1 )

So the start of your statement would look like this

case 
when try_cast(Amount1 as decimal(18,4))  is not null --Numeric 
and try_cast(Amount2 as decimal(18,4)) > 0 --Numeric and not zero
then Concat(CAST((try_cast(Amount1 AS decimal(18,4))/(try_cast(Amount2 AS decimal(18,4))*12))*100 AS decimal(18,2)),'%')