[英]aggregating multiple rows in SQL
For example, I have a table like below -例如,我有一张如下表 -
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id | student_id | section_id | sunday_subj | monday_subj | friday_subj
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1 | U1 | 4 | math | |
2 | U1 | 4 | | biology |
3 | U1 | 4 | | | zoology
4 | U2 | 6 | biology | |
5 | U2 | 6 | | zoology |
6 | U2 | 6 | | | math
Now, from this table I want to achieve the following table -现在,从这张表我想实现下表 -
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id | student_id | section_id | sunday_subj | monday_subj | friday_subj
----------------------------------------------------------------------
1 | U1 | 4 | math | biology | zoology
2 | U2 | 6 | biology | biology | math
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It's not totally clear from your question how you are defining the groups, but here's a guess:从您的问题中并不完全清楚您如何定义组,但这是一个猜测:
SELECT MIN(id) id,
student_id,
section_id,
MAX(sunday_subj) sunday_subj,
MAX(monday_subj) monday_subj,
MAX(friday_subj) friday_subj
FROM MyTable
GROUP BY student_id, section_id;
This at least demonstrates that you can use GROUP BY on two columns.这至少表明您可以在两列上使用 GROUP BY。
All the other columns of your select-list must be inside aggregate functions.选择列表的所有其他列都必须在聚合函数内。
You can group by student_id, section_id
您可以
group by student_id, section_id
set @i = 0;
select
(select @i:=@i+1) id,
t.student_id,
t.section_id,
max(sunday_subj) sunday_subj,
max(monday_subj) monday_subj,
max(friday_subj) friday_subj
from tablename t
group by t.student_id, t.section_id
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