Here's a minor update with a bug fix (to handle the case when the point is at the end of a table field initially):

(defun org-table-mark-field (n)

"Mark the current table field.

If N is negative, select (- N) fields to the left of the current field,

including the current field.

If N >= 2, select (1- N) fields to the right of the current field,

including the current field.

If N is 0 or 1 (default), only the current field is selected."

(interactive "p")

(let ((bol-point (save-excursion

(beginning-of-line)

(point)))

(bof-arg 1)

(eof-arg 1)

(p (point))

bof-p eof-p)

;; Check if the point is already at the beginning of the current field.

(when (looking-back "|\\s-?" bol-point)

(setq bof-p t))

;; Check if the point is already at the end of the current field.

(when (looking-at "\\s-?|")

(setq eof-p t))

;; When selecting current field plus fields to the right

(when (>= n 2)

(setq eof-arg n))

;; When selecting current field plus fields to the left

(when (<= n -1)

(setq bof-arg (- n)))

(org-table-beginning-of-field bof-arg)

(when bof-p

(org-table-next-field))

(set-mark-command nil)

(goto-char p)

(when eof-p

(org-table-beginning-of-field 1))

(org-table-end-of-field eof-arg)

(exchange-point-and-mark)))