How do I put a CASE statement in a SUBSTRING statement? - sql-server

Question

The purpose of this that I'm trying to compare the usernames of two email addresses and see if they're the same. Really, all I want is for this to work.
When I run the query, all I get is: Invalid length parameter passed to the LEFT or SUBSTRING function.
Note: I changed the query. This better illustrates what I'm trying to do.
Note2: I've made the changes so it works properly, but now if the parameter is '', then I get: Invalid length parameter passed to the LEFT or SUBSTRING function.
declare #ReportParameter1 nvarchar(16)
set #ReportParameter1 = 'manmoon#test1.com'
declare #ReportParameter2 nvarchar(16)
set #ReportParameter2 = ''
select 'test'
where SUBSTRING (case #ReportParameter1 when '' then 'x#' else #ReportParameter1 end, 1, Charindex('#', case #ReportParameter1 when '' then 'x#' else #ReportParameter1 end) - 1) =
SUBSTRING (case #ReportParameter2 when '' then 'x#' else #ReportParameter2 end, 1, Charindex('#', case #ReportParameter2 when '' then 'x#' else #ReportParameter2 end) - 1)
Here's the where clause I used to fix the problem. However, this will teach me to be more careful when copying and pasting.
WHERE (substring(#ReportParameter1, 1, case when (CHARINDEX('#', #ReportParameter1) - 1) < 1 then 1 else CHARINDEX('#', #ReportParameter1) - 1 end) = SUBSTRING(#ReportParameter2, 1, CHARINDEX('#', #ReportParameter2) - 1))

Answer 1

The error comes from not comparing the result of the substring to anything. A string is not a boolean expression.
Edit:
Now that you edited your question, that answer doesn't make any sense any more.
I tested your substring expressions with empty strings, and I don't get any error. If you on the other hand have strings that are not empty, but doesn't contain any # character, then you get the error that you describe.
To handle that you could do like this:
... where case
when #ReportParameter1 = '' or charindex('#', #ReportParameter1) = 0 then 'x'
else substring(#ReportParameter1, 1, charindex('#', #ReportParameter1) - 1)
end =
case
when #ReportParameter2 = '' or charindex('#', #ReportParameter2) = 0 then 'x'
else substring(#ReportParameter2, 1, charindex('#', #ReportParameter2) - 1)
end
Note however that two strings that are not email addresses would compare as equal, as 'x' = 'x', so you might want to use different fallback values in the expressions...
Edit 2:
Come to think of it, you don't need to check for empty strings if you check for the # character, as an empty string can't contain a # character:
... where case
when charindex('#', #ReportParameter1) = 0 then 'x'
else substring(#ReportParameter1, 1, charindex('#', #ReportParameter1) - 1)
end =
case
when charindex('#', #ReportParameter2) = 0 then 'x'
else substring(#ReportParameter2, 1, charindex('#', #ReportParameter2) - 1)
end

Answer 2

put the substring in the case... you basically want a static value on one case... on the other ... use the substring on the "else"
case GPS_Quotes.[Sales Engineer]
when
'' then 'some constant value'
else
substring(GPS_Quotes.[Sales Engineer], 1, ...{I don't understand what your are trying to do})
end