行列转换

1. 行转列
   方法一：通过CASE WHEN语句

SELECT 
   username,
   max(CASE WHEN subject = '语文' THEN score END) AS `语文`,
   max(CASE WHEN subject = '数学' THEN score END) AS `数学`,
   max(CASE WHEN subject = '英语' THEN score END) AS `英语`
FROM pivot
GROUP BY username
ORDER BY username;

方法二：通过map_agg函数
通过map_agg函数实现行转列需要以下两个步骤：

通过map_agg函数把两个列的多行的值，映射为map。

 SELECT username, map_agg(subject, score) kv
 FROM pivot
 GROUP BY username
 ORDER BY username;

2. 列转行

方法一：通过UNION语句

 SELECT username, subject, score
FROM (
  SELECT username, '语文' AS subject, yuwen AS score FROM unpivot WHERE yuwen is not null
  UNION
  SELECT username, '数学' AS subject, shuxue AS score FROM unpivot WHERE shuxue is not null
  UNION
  SELECT username, '英语' AS subject, yingyu AS score FROM unpivot WHERE yingyu is not null
)
ORDER BY username;