Query won't convert a nvarchar(30) to a decimal - sql

Question

I have the following query, that when it is run gives the following error:
Conversion failed when converting the nvarchar value '247.300' to data type int.
The query is as follows:
SELECT UD.SerialNumber,
UD.ReadingDate,
UD.ChannelName,
CONVERT(decimal(18,2), ChannelValue) - LAG(CONVERT(decimal(18,2), ChannelValue),1,
CONVERT(decimal(18,2), ChannelValue)) OVER (PARTITION BY ChannelName,
CASE ChannelValue WHEN 0 THEN 0 ELSE 1 END ORDER BY ReadingDate ASC)
FROM [Staging].[UriData] UD
WHERE UD.ChannelName IN (SELECT ChannelName FROM [Staging].[ActiveChannels])
ORDER BY ReadingDate;
Here is a fiddle: DB Fiddle
The only difference between the fiddle (which works fine) and the query here is that I have to have the ChannelValue column NVARCHAR as some of the data will be in text. I have filtered out all of the text data in the query.
I have also tested it with the following query and the data doesn't show up. If it wasn't a number I would expect it to appear here.
select ChannelValue
from [Staging].[UriData]
where try_convert(numeric(18, 2), ChannelValue) is null and ChannelValue is not null;
The expected outcome is that the number '247.300' is converted into a number and processed. Why won't it convert?

Answer 1

If you read the error message it complains about int.
this is from the implicit cast here
CASE ChannelValue WHEN 0 THEN 0 ELSE 1 END
Use an explicit cast
CASE CONVERT(decimal(19,3), ChannelValue) WHEN 0 THEN 0 ELSE 1 END
then values such as 247.300 will convert fine and be comparable to the integer 0.
Though you also say "I have filtered out all of the text data in the query.". In that case you should always use TRY_CONVERT anyway in case operations get ordered such that the conversion happens before the filter.

Answer 2

You cannot easily filter out text values in SQL Server -- the database reserves the right to re-arrange operations and this might cause an error.
So, use TRY_CONVERT():
SELECT UD.SerialNumber,
UD.ReadingDate,
UD.ChannelName,
(TRY_CONVERT(decimal(18, 2), ChannelValue) -
TRY_CONVERT(DECIMAL(18, 2), LAG(ChannelValue), 1, ChannelValue) OVER (PARTITION BY ChannelName, CASE ChannelValue WHEN '0' THEN 0 ELSE 1 END ORDER BY ReadingDate ASC))
)
FROM [Staging].[UriData] UD
WHERE UD.ChannelName IN (SELECT ChannelName
FROM [Staging].[ActiveChannels]
)
ORDER BY ReadingDate;
If ChannelValue is a string, then the comparison in the CASE should be to a string value.
Under most circumstances, you would only need to do the conversion once. However, to avoid a problem with the subtraction, you also have to convert before the subtraction.
You can use this logic to simplify:
SELECT UD.SerialNumber,
UD.ReadingDate,
UD.ChannelName,
(ChannelValue_d -
LAG(ChannelValue_d), 1, ChannelValue_d) OVER (PARTITION BY ChannelName, CASE ChannelValue WHEN '0' THEN 0 ELSE 1 END ORDER BY ReadingDate ASC)
)
FROM [Staging].[UriData] UD CROSS APPLY
(VALUES (TRY_CONVERT(decimal(18, 2), ChannelValue) )
) v(ChannelValue_d)
WHERE UD.ChannelName IN (SELECT ChannelName
FROM [Staging].[ActiveChannels]
)
ORDER BY ReadingDate;