Oracle 行转列总结 Case When,Decode,PIVOT 三种方式

最近又碰到行专列问题了,当时不假思索用的是子查询,做完后我询问面试管行专列标正的写法应该如何写,他告诉我说应该用"Decode",索性我就总结一下,一共三种方式 

--=========建表语句 

CREATE TABLE populations( 
country VARCHAR2(20), 
sex INT, 
populcation INT 
); 

--=========插入记录 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('中国  ', 1, 340); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('中国  ', 2, 260); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('美国  ', 1, 45); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('美国  ', 2, 55); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('加拿大', 1, 51); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('加拿大', 2, 49); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('英国  ', 1, 40); 

insert into populations (COUNTRY, SEX, POPULCATION) 
values ('英国  ', 2, 60); 

commit; 

--=========三种方式 

-------------------------------------- 

Decode 方式 

SELECT a.COUNTRY AS "国家", 
   SUM(DECODE(a.SEX, 1, a.POPULCATION)) AS "男", 
   SUM(DECODE(a.SEX, 2, a.POPULCATION)) AS "女" 
  FROM populations a 
GROUP BY a.COUNTRY 

-------------------------------------- 

Case When 方式 

SELECT a.COUNTRY AS "国家", 
   SUM(CASE 
WHEN a.SEX = 1 THEN 
  a.POPULCATION 
   END) AS "男", 
   SUM(CASE 
WHEN a.SEX = 2 THEN 
  a.POPULCATION 
   END) AS "女" 
  FROM populations a 
GROUP BY a.COUNTRY 

-------------------------------------- 

PIVOT 方式 11G特有的方式 

SELECT * 
  FROM (SELECT a.COUNTRY AS "国家", a.SEX, a.POPULCATION FROM populations a) 
  PIVOT( 
SUM(POPULCATION) 
FOR SEX IN(1 AS "男",2 AS "女"))