方法1：
SELECT 
CASE WHEN to_number(to_char(sysdate,'mm'),99) <4 then '春季' 
when to_number(to_char(sysdate,'mm'),99) between 4 and 6 then '夏季'
when to_number(to_char(sysdate,'mm'),99) between 7 and 9 then '秋季'  
ELSE '冬季' END CASE 
from dual

方法2：
select trunc(sysdate,'q') from dual