oracle case when的用法

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 &#39;1&#39;  
WHEN salary > 500 AND salary <= 600  THEN &#39;2&#39;  
WHEN salary > 600 AND salary <= 800  THEN &#39;3&#39;  
WHEN salary > 800 AND salary <= 1000 THEN &#39;4&#39;  
ELSE NULL END salary_class, -- 别名命名
COUNT(*)  
FROM    Table_A  
GROUP BY  
CASE WHEN salary <= 500 THEN &#39;1&#39;  
WHEN salary > 500 AND salary <= 600  THEN &#39;2&#39;  
WHEN salary > 600 AND salary <= 800  THEN &#39;3&#39;  
WHEN salary > 800 AND salary <= 1000 THEN &#39;4&#39;  
ELSE NULL END;