Leetcode刷题记之学生地理信息报告

学生地理信息报告：实现对大洲(continent)列的透视表操作

题目来源：Leetcode 618

什么叫做对continent列的透视表操作？

即，按照continent列对学生信息进行汇总，

期望输出结果：

对照原数据表，可以明确一个大致的解题思路：行转列

① 尝试行转列

SELECT (CASE WHEN continent='America' THEN name ELSE Null END) AS 'America',
       (CASE WHEN continent='Asia' THEN name ELSE Null END) AS 'Asia',       
       (CASE WHEN continent='Europe' THEN name ELSE Null END) AS 'Europe'      
FROM student

输出结果：

看来行转列的思路没错，可是比我们的预期结果要多了那么一些null...

② 尝试聚合行

SELECT MAX((CASE WHEN continent='America' THEN name ELSE Null END)) AS 'America',
       MAX((CASE WHEN continent='Asia' THEN name ELSE Null END)) AS 'Asia',       
       MAX((CASE WHEN continent='Europe' THEN name ELSE Null END)) AS 'Europe'      
FROM student

输出结果：

Umm...好像比我们的预期结果又少了点什么....

MAX只能取到最大值，所以Jack...丢了...

③ 尝试以序列来分组聚合

SELECT name,continent,
	IF(@con=continent,@row_num:=@row_num+1,@row_num:=1) rank,
       @con:=continent
FROM student,(SELECT @con:=null,@row_num:=1) a
ORDER BY continent,name

输出结果：

Jack and Jane，有点完美。(千万记得order by，不然...)

④ 完整步骤

SELECT MAX((CASE WHEN continent='America' THEN name ELSE Null END)) AS 'America',
       MAX((CASE WHEN continent='Asia' THEN name ELSE Null END)) AS 'Asia',       
       MAX((CASE WHEN continent='Europe' THEN name ELSE Null END)) AS 'Europe'      
FROM (
	SELECT name,continent,
		IF(@con=continent,@row_num:=@row_num+1,@row_num:=1) rank,
		@con:=continent
	FROM student,(SELECT @con:=null,@row_num:=1) a
	ORDER BY continent,name) b
GROUP BY rank

输出结果：

DONE !