How to repeat this piece of code with a macro:

,@(if (/= (length (parameter "ime")) 0) (list 'ime (parameter "ime"))  (list
'ime :null))

while 'ime and "ime" changes while repeating.

That's already a part of a "sql-compile" macro, like this:

(sql-compile `(:insert-into 'kupci :set
,@(if (/= (length (parameter "ime")) 0) (list 'ime (parameter "ime"))  (list
'ime :null))
,@(if (/= (length (parameter "prezime")) 0) (list 'prezime (parameter
"prezime")) (list 'prezime
and so on.

I should be able to call it like: (a-macro 'ime 'prezime ...)

,@ outside of ` is meaningless.

(defun generate-sql-set (parameter)
  (let* ((name  (string parameter))
         (value (parameter name)))
    (list parameter
          (if (zerop (length value))
              :null
              value))))

(defun generate-sql-sets (parameters)
  (reduce (function append) parameters
          :key (function generate-sql-set)
          :from-end t))

(sql-compile `(:insert-into 'kupci :set ,(generate-sql-sets '(ime prezime))))