The function `sql-add-product' adds an element corresponding to a
sql-product P to the alist `sql-product-alist', with P as
intended key.  But the level of nesting in the call to
add-to-list is one to deep, as should be obvious from the snippet
below.

sql.el:
(defun sql-add-product (product display &rest plist)
  [...]
  ;; Don't do anything if the product is already supported
  (if (assoc product sql-product-alist)
      (user-error "Product `%s' is already defined" product)
    ;; Add product to the alist
    (add-to-list 'sql-product-alist `((,product :name ,display . ,plist)))
    [...]))

-ap