Overcoming SQL's lack of short-circuit of AND conditions - sql-server

Question

I'm processing an audit log, and want to ignore entries where a NULL value is changed to zero (or remains NULL). The Old and New values are held in NVARCHAR fields regardless of the type of the fields being logged. In order to CAST a new value to decimal, to determine if it's zero, I need to restrict to cases where ISNUMERIC of the field returns 1.
I've got it to work with this strange bit of SQL - but feel sure there must be a better way.
WHERE MessageDesc LIKE 'poitem%'
AND NOT(OldValue IS NULL AND 0.0 =
CASE
WHEN ISNUMERIC(NewValue) = 1 THEN CAST(NewValue AS DECIMAL(18,4))
WHEN NewValue IS NULL THEN 0.0
ELSE 1.0
END)
Any suggestions?

Answer 1

SQL Server 2012 added a Try_Convert function, which returns NULL if the value cannot be casted as the given type. http://technet.microsoft.com/en-us/library/hh230993.aspx
WHERE NOT (OldValue is Null AND
(NewValue is null OR try_convert(float, NewValue) = 0.0)
)
If using a version prior to 2012, check out Damien_The_Unbeliever's answer here: Check if a varchar is a number (TSQL) ...based on Aaron's comment this will not work in all cases.
Since you are using SQL 2008, then it appears a combination of isnumeric and a modified version of Damien's answer from the link above will work. Your current solution in your question would have problems with values like '.', '-', currency symbols ($, etc.), and scientific notation like '1e4'.
Try this for SQL 2008 (here is SQLFiddle with test cases: http://sqlfiddle.com/#!3/fc838/3 ): Note: this solution will not convert text values to numeric if the text has commas (ex: 1,000) or accounting notation with parens (ex: using "(1)" to represent "-1"), because SQL Server will throw an error when trying to cast to decimal.
WHERE t.OldValue is null
AND
(
t.NewValue is null
OR
0.0 =
case
when isnumeric(t.NewValue) = 1
--Contains only characters that are numeric, negative sign, or decimal place.
--This is b/c isnumeric will return true for currency symbols, scientific notation, or '.'
and not (t.NewValue like '%[^0-9.\-\+]%' escape '\')
--Not other single char values where isnumeric returns true.
and t.NewValue not in ( '.', '-', '+')
then cast(t.NewValue as decimal(18,4))
else null --can't convert to a decimal type
end
)

Answer 2

Avoid ISNUMERIC() since it is problematic with '.'.
-- Dot comes up as valid numeric
select
ISNUMERIC('.') as test1,
ISNUMERIC('1A') as test2,
ISNUMERIC('1') as test3,
ISNUMERIC('A1') as test4
-- Results window (text)
test1 test2 test3 test4
----------- ----------- ----------- -----------
1 0 1 0
Use COALESCE() instead.
WHERE MessageDesc LIKE 'poitem%'
AND
NOT (OldValue IS NULL AND
CAST(COALESCE(NewValue, '0') AS DECIMAL(18,4)) = 0)