oracle case when 语句的用法详解

--简单Case函数 
CASE sex 
WHEN '1' THEN '男' 
WHEN '2' THEN '女' 
ELSE '其他' END 
--Case搜索函数 
CASE
WHEN sex = '1' THEN '男' 
WHEN sex = '2' THEN '女' 
ELSE '其他' END 

SELECT   grade, COUNT (CASE WHEN sex = 1 THEN 1      /sex 1为男生，2位女生/
                       ELSE NULL
                       END) 男生数,
                COUNT (CASE WHEN sex = 2 THEN 1
                       ELSE NULL
                       END) 女生数
    FROM students GROUP BY grade;

SELECT T2., T1.
   FROM T1, T2
  WHERE (CASE WHEN T2.COMPARE_TYPE = 'A' AND
                   T1.SOME_TYPE LIKE 'NOTHING%'
                THEN 1
              WHEN T2.COMPARE_TYPE != 'A' AND
                   T1.SOME_TYPE NOT LIKE 'NOTHING%'
                THEN 1
              ELSE 0
           END) = 1

SELECT 
CASE WHEN salary <= 500 THEN '1' 
WHEN salary > 500 AND salary <= 600  THEN '2' 
WHEN salary > 600 AND salary <= 800  THEN '3' 
WHEN salary > 800 AND salary <= 1000 THEN '4' 
ELSE NULL END salary_class, -- 别名命名
COUNT() 
FROM    Table_A 
GROUP BY 
CASE WHEN salary <= 500 THEN '1' 
WHEN salary > 500 AND salary <= 600  THEN '2' 
WHEN salary > 600 AND salary <= 800  THEN '3' 
WHEN salary > 800 AND salary <= 1000 THEN '4' 
ELSE NULL END; 

select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from   employees;

SELECT T2., T1.
   FROM T1, T2
  WHERE (T2.COMPARE_TYPE = 'A' AND T1.SOME_TYPE LIKE 'NOTHING%')
         OR
        (T2.COMPARE_TYPE != 'A' AND T1.SOME_TYPE NOT LIKE 'NOTHING%')