橫豎表相互轉換SQL

以Oracle數據庫爲例，測試在數據處理中經常遇到數據庫中橫表和豎表的相互轉換，採用學生各科成績分數爲測試數據

橫錶轉換豎表:

CREATE table score
(
   username varchar(10) ,
   chinese  int ,
   math     int ,
   physics  int
);
 
INSERT into score(username , chinese , math , physics) values('Kay',74,83,93);
INSERT into score(username , chinese , math , physics) values('Jim',75,84,94);
INSERT into score(username , chinese , math , physics) values('Lily',86,49,91);
 
COMMIT;
 
SELECT * FROM score;

橫錶轉換成豎表關鍵在於如何把一條記錄轉換成多條記錄，三個分數字段對應三條記錄，可以採用一個只有三條記錄中間表和score表做笛卡爾積把一條記錄copy成三條記錄

WITH T AS
( SELECT 'chinese' course FROM dual
  UNION ALL
  SELECT 'math'    course FROM dual
  UNION ALL 
  SELECT 'physics' course FROM dual
) 
SELECT username,course,decode(course,'chinese',chinese,'math',math,'physics',physics) 
  FROM score ,T  ORDER BY username ,course;

豎錶轉換橫表：

SELECT * FROM course;

豎錶轉換橫表關鍵在於如何把多條記錄合併成一條記錄，按照username進行group by 使用聚合函數把多條合併成一條且按不同的類別值合併到不同的字段

SELECT username, 
       MAX(decode(course,'chinese',score)) chinese,
       MAX(decode(course,'math',score)) math,
       MAX(decode(course,'physics',score)) physics
  FROM course
GROUP BY username
ORDER BY username