Casting Scientific Notation (from varchar -> numeric) in a view

you can use ISO "real" datatype

SELECT convert(numeric(18,18),convert(real,'820.0E-12'))
--OR with more precision
SELECT convert(numeric(18,18),convert(float(53),'820.0E-12'))

There's a couple different problems all coming together here at the same time. Let's look at some of them:

1. You're casting numbers as DECIMAL(18, 18). What that means is "give me a number that has room for a TOTAL of 18 characters, and 18 of them should be after the decimal". That works fine as long as your number is smaller than 0 (which is true for all E- numbers) but it will break if you try to use it on numbers > 0. For numbers > 0, just cast as DECIMAL without specifying anything else.

2. In the case where you add "WHEN @d like '%E+%' THEN CAST(@d AS FLOAT)", you're getting different results for numbers < 0 because the engine is implicitly casting the result differently. I don't know the rules on how sql server decides to cast CASE results, but apparently making your proposed change causes the engine to recast it in a different way. Explicitly casting those results as decimal fixes the issue.

3. You need to LTRIM and RTRIM your results consistently. You can either add LTRIM and RTRIM to each case statement, or you can just LTRIM and RTRIM the results of the case.

Here's a solution that should totally solve everything:

SELECT
    LTRIM(RTRIM(CASE 
        WHEN @d like '%E-%' THEN CAST(CAST(@d AS FLOAT) AS DECIMAL(18,18))
        WHEN @d like '%E+%' THEN CAST(CAST(@d AS FLOAT) AS DECIMAL)
        ELSE @d
    END))