Perhaps I'm not explaining clearly enough.

The comparison to the customer ID is hard coded in the WHERE clause. It has nothing to do with the "Customer_ID = " that you initially put in the @WHERE_CLAUSE variable, because you are parsing that out.

You can change the value of @WHERE_CLAUSE to start with "ANYCOLUMN = 0000000008" and the comparison will still be done against the Customer_ID column.

Try altering the second line of your code and see.

-- This works and will always work, but only as a test against the customer_id

DECLARE

@SQL VARCHAR(1000)

, @WHERE_CLAUSE VARCHAR(1000) = 'NOTHING TO THE LEFT OF THE EQUAL SIGN MATTERS = 0000000008'

SELECT

*

FROM

dbo.TEST AS C

WHERE

CUSTOMER_ID = (

CASE

WHEN @WHERE_CLAUSE IS NOT NULL

THEN LTRIM(RTRIM(SUBSTRING(@WHERE_CLAUSE, PATINDEX('%=%', @WHERE_CLAUSE)+1, LEN(@WHERE_CLAUSE))))

ELSE '1'

END)