GROUP BY clause and CASE statement – need help

I have a table with 4 columns

Table name = products
Column name = product_id, product_programname, product_title, model_number

When I use GROUP BY with model_number, it alway returns me the info of product (ebay) which comes first in the queue.
But i want to get the title from 'argos' if not null else 'buy' if not null etc.

The below query doesn't works for me.

SELECT *,
(
case(true)
WHEN product_programname = 'argos' AND product_programname IS NOT NULL
THEN product_title

WHEN product_programname = 'buy' AND product_programname IS NOT NULL
THEN product_title

WHEN product_programname = 'amazon' AND product_programname IS NOT NULL
THEN product_title

WHEN product_programname = 'ebay' AND product_programname IS NOT NULL
THEN product_title

WHEN product_programname = 'next' AND product_programname IS NOT NULL
THEN product_title

END
) as final_title

FROM products
GROUP BY model_number

Code (markup):

Please assist how come it'll be possible.

 

Just use COALESCE.
This is incorrect usage of group by clause :