CREATE TABLE "time_of_year"
(
    "year" (smallint)
    ,"month" (smallint)
    ,"season" (conditional {case expression} )
)
;

CASE "month"
WHEN "month" in (12,1,2) THEN 'winter'
WHEN "month" in (3,4,5) THEN 'spring'
WHEN "month" in (6,7,8) THEN 'summer'
ELSE 'fall'
END