Control sort by in postgres SQL using parameter [duplicate]

EDIT: I found a solution which I provided at the bottom.

I have the following postgres sql function:

CREATE OR REPLACE FUNCTION public.get_products(_sort_by TEXT DEFAULT 'DEFAULT'::TEXT)
 RETURNS SETOF products
 LANGUAGE sql
 STABLE
AS $function$
SELECT
  p.*
from
  products p
 ORDER BY p.is_mattress DESC, p.sale, p.regular
  
$function$;

This works. What I am trying to do is control the sort by logic depending on what _sort_by is when it comes in.

IE: _sort_by = 'DEFAULT' I want the sort by to be p.is_mattress DESC, p.sale, p.regular _sort_by = 'PRICE_LOW_TO_HIGH' would be a sort by of COALESCE(p.sale, p.regular) ASC

I am trying to get CASE to work here since it seems logically equivalent to what I want to do. this is my best attempt but this isnt resulting in an error so obviously isnt correct.

The error: ERROR: syntax error at or near "DESC" LINE 68: WHEN 'DEFAULT' THEN p.is_mattress DESC, p.sale, p.re...

CREATE OR REPLACE FUNCTION public.get_products(_sort_by TEXT DEFAULT 'DEFAULT'::TEXT)
 RETURNS SETOF products
 LANGUAGE sql
 STABLE
AS $function$
SELECT
  p.*
from
  products p
 ORDER BY 
    CASE _sort_by
        WHEN 'DEFAULT' THEN p.is_mattress DESC, p.sale, p.regular
        WHEN 'PRICE_LOW_TO_HIGH' THEN COALESCE(p.sale, p.regular) ASC
        WHEN 'PRICE_HIGH_TO_LOW' THEN COALESCE(p.sale, p.regular) DESC
    END
$function$;

The attached question was not what I was looking for. However, I was able to solve with the following query:

CREATE OR REPLACE FUNCTION public.get_products(_sort_by TEXT DEFAULT 'DEFAULT'::TEXT)
 RETURNS SETOF products
 LANGUAGE sql
 STABLE
AS $function$
SELECT
  p.*
from
  products p
  ORDER BY 
   CASE WHEN _sort_by = 'PRICE_LOW_TO_HIGH' THEN COALESCE(p.sale, p.regular) END ASC,
   CASE WHEN _sort_by = 'PRICE_HIGH_TO_LOW' THEN COALESCE(p.sale, p.regular) END DESC,
   CASE WHEN _sort_by = 'BRAND_A_TO_Z' THEN b.name END ASC,
   CASE WHEN _sort_by = 'BRAND_Z_TO_A' THEN b.name END DESC,
   CASE WHEN _sort_by = 'DEFAULT' THEN p.is_mattress END DESC,
   CASE WHEN _sort_by = 'DEFAULT' THEN p.sale END,
   CASE WHEN _sort_by = 'DEFAULT' THEN p.regular END
$function$;