1. 源表結構：

order_id	class	count
111101100002	3	0
111101100012	1	3
11110112002	1	2
111101100202	2	0

2. 目標表結構：

根據order_id分組，根據不同的class進行統計

order_id	count_1	count_2		count_3	count_4		count_5	count_6		count_7	count_8		count_9	count_10
111101100002	3	0	0	3	0	0	3	0	0	3	0	0	3	0

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3. sql語句如下：

（1）分組統計，
（2）使用 COALESCE 返回第一個不為null的值，如果值為null，返回0

select order_id,
sum(COALESCE(cass class when 1 then count END,0)) as count_1,
sum(COALESCE(cass class when 2 then count END,0)) as count_2,
sum(COALESCE(cass class when 3 then count END,0)) as count_3,
sum(COALESCE(cass class when 4 then count END,0)) as count_4,
sum(COALESCE(cass class when 5 then count END,0)) as count_5,
sum(COALESCE(cass class when 6 then count END,0)) as count_6,
sum(COALESCE(cass class when 7 then count END,0)) as count_7,
sum(COALESCE(cass class when 8 then count END,0)) as count_8,
sum(COALESCE(cass class when 9 then count END,0)) as count_9,
sum(COALESCE(cass class when 10 then count END,0)) as count_10
from test_table group by order_id;

4. IF ELSE不太好用，可以用CASE WHEN THEN ELSE代替

比如 active_hour int 值分別為0~23
但是想要得到倆位的HH的小時值，需要把 0~9 轉換為 00 ~ 09

select active_hour,case when active_hour > 9 then ''||active_hour else '0'||active_hour from t_active;

總結

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