Error with complex Oracle query

You have a WHERE clause outside of the subquery in your SELECT declaration.
I color coded it for you and explained that your footer code ADDs a new WHERE to the SQL.

You're misusing the SELECT by having a FROM and a WHERE in it.  The CI code expects that you'll use the SELECT for SELECT statements (and the FROM and WHERE clauses for those respective functions) - you're using it wrong.

Use it as designed and this problem will go away.