An indentation heuristic for Emacs' sql-mode

indent_sql.el

(defun get-previous-indentation ()
  "Get the column of the previous indented line"
  (interactive)
  (save-excursion
    (progn
      (move-beginning-of-line nil)
	  (skip-chars-backward "\n \t")
      (back-to-indentation))
    (current-column)))

(defun get-current-indentation ()
  "Return column at current indentation"
  (interactive)
  (save-excursion
    (progn
      (back-to-indentation)
      (current-column))))

(defun point-at-current-indentation ()
  "Return point at current indentation"  
  (interactive)
  (save-excursion
    (progn
	  (move-to-column (get-current-indentation))
      (point))))

(defun point-at-column-on-line (col)
  "Returns the point at `col` on the current line"
  (interactive)
  (save-excursion
    (progn
      (move-to-column col)
      (point))))

(defun ig-move-line-to-column (col)
  "Move the line to col; fill with all spaces if moveing forward"
 (interactive "p")
  (let ((point-at-cur-indent (point-at-current-indentation))
		(col-at-cur-indent (get-current-indentation)))
    (cond (
		   (= col 0)
		   ;; delete to beginning of line or do nothing
		   (if (= col-at-cur-indent 0)
			   nil
			 (delete-region point-at-cur-indent (point-at-column-on-line 0))))
		  (
			  (< col col-at-cur-indent)
			  ;; delete from our current point BACK to col
			  (delete-region (point-at-column-on-line col) point-at-cur-indent))
		  (
		   (> col col-at-cur-indent)
		   ;; delete all text from indent to beginning of line
		   (progn
			 (delete-region point-at-cur-indent (point-at-column-on-line 0))
			 (move-beginning-of-line nil)
			 ;; add spaces forward
			 (insert-string (make-string col ?\s)))))))

(defun ig-indent-sql ()
  "Indent by `tab-width` at most 1 time greater than the previously indented line otherwise go to the beginning of the line indent forward by `tab-width`"
  (let ((previous (get-previous-indentation))
        (current (get-current-indentation)))
    (cond ( ;; exactly at previous line's indentation
           (= previous current)
		   (ig-move-line-to-column (+ current tab-width)))

	  ( ;; current is greater than previous
	   (> current previous)
	    ;; exactly at one indentation forward from previous lines indent
	   (if (= tab-width (- current previous))
		;; move line to beginning
	       (ig-move-line-to-column 0)
	     ;; go back to previous indentation level
	     (ig-move-line-to-column previous)))

          (t 
	   (ig-move-line-to-column (+ current tab-width))))))


(add-hook 'sql-mode-hook
          (function (lambda ()
                      (make-local-variable 'indent-line-function)
                      (setq indent-line-function 'ig-indent-sql))))

I ported this to Emacs 26.1 in this fork:

https://gist.github.com/royseto/91e6db52e58a71ed878c1ac5e039794d

Emacs no longer seems to define insert-string, so I replaced (insert-string (make-string col ?\s)) with (insert-char ?\s col).

I also set tab-width to 4 in SQL mode per my preference.