[nqk]

Hic, I know nothing about codes.
The best I could do to this is:

program:
val = field('#pages');
first_non_empty(
	cmp(val, 6, 'very short (~90)', '', ''),
	cmp(val, 11, 'short (91-200)', '', ''),
	cmp(val, 16, 'medium (201-450)', '', ''),
	cmp(val, 21, 'large (451-1000)', '', ''),
	'very large (1001~)');

I don't understand the figures right after val.

In my case, #pages include number of pages.

I created a new custom column based on other column and pasted the code to the box underneath that. (I think I did it wrong).

Please correct me.

Edit: Forget it, I figured it out. . Done.
It became like this

program:
val = field('#pages');
first_non_empty(
	cmp(val, 90, 'Rất ngắn (~ 89)', '', ''),
	cmp(val, 250, 'Ngắn (90-249)', '', ''),
	cmp(val, 550, 'Vừa (250-549)', '', ''),
	cmp(val, 1200, 'Dài (550-1199)', '', ''),
	'Rất dài (1200 ~)');