not a group by expression case statement
SELECT a, b, sum(c) FROM my_table GROUP BY a; -- Error
SELECT a, b, sum(c) FROM my_table GROUP BY a, b; -- OK, all expressions there
-- CASE WHEN statement, for example:
SELECT a, CASE WHEN b=1 THEN 'Ok' ELSE 'Ko' END AS r, sum(c)
FROM my_table
GROUP BY a, CASE WHEN b=1 THEN 'Ok' ELSE 'Ko' END; -- No alias name (r)