我有一个这样的表（表1）（我简化了它）

code   |    description   |    season    
----------+------------------+--------------
   500    |      info 1      |    fall    
   500    |      info 4      |    fall    
   500    |      info 8      |    fall    
   500    |      info 1      |    winter
   300    |      info 1      |    spring
   400    |      info 1      |    fall

我想要一个如下的表格，在那里我有每个季节的代码频率

season  |  Number of Unique Codes  
----------+------------------------
   fall   |           2      
  winter  |           1           
  spring  |           1

到目前为止，我有：

SELECT 
    season, 
    count(DISTINCT code) AS "Number of Unique Codes"
FROM table_1
WHERE code IS NOT NULL
GROUP BY season
ORDER BY code desc;

然而，我遇到了一些问题。