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
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