通用写法 

SELECT 
USER_ACCOUNT,
(CASE 
  WHEN SEX = 1 THEN '男' 
  WHEN SEX = 2 THEN '女' 
  ELSE '未知'
END )sex
FROM m_user u

Oracle特有写法

SELECT 
   USER_ACCOUNT,
   decode(sex,1,'男',2,'女','未知') "sex"
FROM m_user u

     再提供一个判断个数的示例

SELECT DECODE(COUNT(1),0,'','有调课') from 表