樞行列基於條件

Question

我有一個表像這樣

id student_id score 
1 1   45 
2 2   55 
3 2   75 
4 3   80 
5 1   90 
6 2   78 
7 3   55 
8 1   45 
9 1   65 

我想這樣

student_id s1 s2 s3 s4 
1   45 90 45 65 
2   55 75 78 - 
3   80 55 - - 

樞紐的概念來安排它是

SELECT 
    item_id, 
    MAX(IF(property_name = 'property_value1', value, NULL)) AS alias1, 
    MAX(IF(property_name = 'property_value2', value, NULL)) AS alias2, 
    ... 
    ... 
    ... 
FROM 
    table 
GROUP BY 
    item_id; 

這我無法真正瞭解我的情況，因爲我正在創建s1 - s4列，即每個學生的第一個分數變爲s1，第二個成爲S2等

我如何解決這個

Answer 1

最簡單的方法是將值一列：

select student_id, group_concat(score order by id) 
from t 
group by student_id; 

足以用於多種用途。如果你想分開的列，你需要創建一個列。一種方法是使用變量：

select student_id, 
     max(case when rn = 1 then score end) as score_1, 
     max(case when rn = 2 then score end) as score_2, 
     max(case when rn = 3 then score end) as score_3, 
     max(case when rn = 4 then score end) as score_4 
from (select t.*, 
      (@rn := if(@s = student_id, @rn + 1, 
         if(@s := student_id, 1, 1) 
         ) 
      ) as rn 
     from t cross join 
      (select @s := -1, @rn := 0) params 
     order by student_id, id 
    ) t 
group by student_id;