What database are you connecting to? Most of the time you'll want to be using single quotes instead of double quotes, unless you're connecting to something like Access.
Does the same SQL work when you execute it directly against the database in question outside of Tableau?
Hi I am connecting it to Access. It's funny how the LEFT and INSTR function works on Access, but doesn't work on Toad (another SQL tool I use).
Out of curiosity, do different tools have different set of "SQL functions"?
While syntax can be different for each database's implementation of instr(), your syntax is not right -- at least for Access.
Your instr is only using two arguments:
You're essentially beginning to ask the question: "Starting with the first character in the string Request ID....", but you don't give Instr a 3rd argument which is telling it what to look for. Your expression shouldn't work in Access at all.
You want something like instr(1,[FieldName], "string to search for inside [FieldName]"), I think.
I'm not sure I understand your last question. Can you re-phrase?