How can I use data from a string in SQL in a numeric comparison? - sql

Question

I'm a B-grade SQL user, so bear with me. I have a field that is in the NVARCHAR format ("Year"), but all but only about 1 in 1000 records is something other than a number. Yes, this is a ridiculous way to do this, but we receive this database from a customer, and we can't change it.
I want to pull records from the database where the year field is greater than something (say, 2006 or later). I can ignore any record whose year doesn't evaluate to an actual year. We are using SQL server 2014.
I have created an embedded query to convert the data to a "float" field, but for whatever reason, I can't add a where clause with this new floating-point field. I originally tried using a "case-if" but I got the same result.
I'm pulling my hair out, as I'm either missing something really silly, or there's a bug in SQL server. When I look at the field in the little hint, it's showing as a float. When I run this, I get "Error converting data type nvarchar to float."
SELECT VL.Field_A,
VL.FLYear,
VL.Field_B
FROM
(select
Field_A,
cast ([Year] as float) as FLYear,
/* didn't work either*/
/*Convert(float, [Year]) as FLYear, */
Field_B
from CustomerProvidedDatabaseTable
where (Field_A like 'E-%' OR
Field_A like 'F-%')
and
(isnumeric(year)=1)
and
year is not null
) VL
/* this statement is the one it chokes on */
where
VL.FLYear >= 2006.0
If I remove the last "where" clause, it works fine, and the field looks like a number. If I change the last where clause to:
where VL.FLYear like '%2006%'
SQL Server accepts it, though of course it doesn't return me all the records I want.

Answer 1

Try to simplify it and just use TRY_CONVERT(DATETIME, aYearvalue) or TRY_PARSE which will return NULL for values it can't convert and continue to process valid rows. I think you can do away with the where clause as join and just work directly against the column like: (substitute the literal string after datetime with your column)
SET DATEFORMAT mdy;
Select YEAR(try_convert(datetime, '08/01/2017')) as value1
WHERE value1 >=2016;

Answer 2

Try cast/convert to a numeric data type. I have modified the last line of your query to do just that. Take a peek.
SELECT
VL.Field_A,
VL.FLYear,
VL.Field_B
FROM
(select
Field_A,
cast ([Year] as float) as FLYear,
/* didn't work either*/
/*Convert(float, [Year]) as FLYear, */
Field_B
from CustomerProvidedDatabaseTable
where (Field_A like 'E-%' OR
Field_A like 'F-%')
and
(isnumeric(year)=1)
and
year is not null
) VL
/* this statement is the one it chokes on */
where
ISNUMERIC(VL.FLYear) = 1
and
CAST(VL.FLYear AS INT) >= 2006
Check out the following link for cast and convert documentation:
https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql
NOTE: ISNUMERIC will return true ( a false positive for a value which has a scientific numerical value, e.g. 1E10, though I don't see this happening from your data).
Another option is TRY_CONVERT.
Documentation on TRY_CONVERT: https://learn.microsoft.com/en-us/sql/t-sql/functions/try-convert-transact-sql

Answer 3

Try using Cast . Use the below link to check in more detail about casting.
https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql