在Oracle中，数据库中有一张如下所示的表，表名为sales。。。查询出如下所示的结果。【行转列】

CREATE TABLE sales(年 NUMBER,季度 NUMBER	,销售量 NUMBER);
INSERT INTO sales VALUES(1991, 1 ,11);
INSERT INTO sales VALUES(1991, 2 ,12);
INSERT INTO sales VALUES(1991, 3 ,13);
INSERT INTO sales VALUES(1991, 4 ,14);
INSERT INTO sales VALUES(1992, 1 ,21);
INSERT INTO sales VALUES(1992, 2 ,22);
INSERT INTO sales VALUES(1992, 3 ,23);
INSERT INTO sales VALUES(1992, 4 ,24);

SELECT * FROM sales;

------------------------------------
   	年	季度	销售量
1	1991	1	11
2	1991	2	12
3	1991	3	13
4	1991	4	14
5	1992	1	21
6	1992	2	22
7	1992	3	23
8	1992	4	24

SELECT 年, 
	SUM(CASE WHEN 季度=1 THEN 销售量 ELSE 0 END) AS 一季度,
	SUM(CASE WHEN 季度=2 THEN 销售量 ELSE 0 END) AS 二季度,
	SUM(CASE WHEN 季度=3 THEN 销售量 ELSE 0 END) AS 三季度,
	SUM(CASE WHEN 季度=4 THEN 销售量 ELSE 0 END) AS 四季度
FROM SALES 
GROUP BY 年;

SELECT *
  FROM SALES
PIVOT(SUM(销售量)
   FOR 季度 IN(1 AS "一季度", 2 AS "二季度", 3 AS "三季度", 4 AS "四季度"))
 ORDER BY 1;

SELECT T.年,
       NVL(SUM(T1.一季度),0) AS "一季度",
       NVL(SUM(T2.二季度),0) AS "二季度",
       NVL(SUM(T3.三季度),0) AS "三季度",
       NVL(SUM(T4.四季度),0) AS "四季度"
  FROM (SELECT 年,销售量 AS "一季度" FROM SALES A WHERE A.季度 = '1') T1,
       (SELECT 年,销售量 AS "二季度" FROM SALES A WHERE A.季度 = '2') T2, 
       (SELECT 年,销售量 AS "三季度" FROM SALES A WHERE A.季度 = '3') T3, 
       (SELECT 年,销售量 AS "四季度" FROM SALES A WHERE A.季度 = '4') T4,
       (SELECT DISTINCT 年 FROM SALES) T
 WHERE T.年 = T1.年(+)
   AND T.年 = T2.年(+)
   AND T.年 = T3.年(+)
   AND T.年 = T4.年(+)
 GROUP BY T.年
 ORDER BY 1;