枢轴查询不起作用 [英] pivot query doesn't work

问题描述

我有生以来第一次尝试在我的数据库表上使用 PIVOT.这是表:

I'm trying for the first time in my life to use the PIVOT on my database table. This is the table:

Answers (id_user, id_question, topic, value) VALUES
(1, 1, 'a', 3),
(1, 1, 'b', 3),
(1, 1, 'c', 2),

(1, 2, 'a', 1),
(1, 2, 'b', 2),
(1, 2, 'c', 3),

[...]

(1, 14, 'a', 1),
(1, 14, 'b', 2),
(1, 14, 'c', 1);

但我正在努力做到这一点:

But I'm trying to get to this:

Answers (id_user, id_question, topic_A, topic_B, topic_C) VALUES
(1, 1, 3, 3, 2),
(1, 2, 1, 2, 3),
[...]
(1, 14, 1, 2, 1);

通过这个查询，我在 PIVOT 附近得到一个通用语法错误

with this query I get a generic syntax ERROR near PIVOT

SELECT *
FROM
(     SELECT id_user, id_question, topic, value
      FROM Answers
      WHERE id_user=98
) as risp
PIVOT 
(     FOR id_question IN ([a],[b],[c])
)     AS pvt

推荐答案

在 MySQL 中使用聚合的一种方法:

One way todo this in MySQL using aggregation:

select id_user, id_question,
       max(case when topic = 'A' then value end) as topic_A,
       max(case when topic = 'B' then value end) as topic_B,
       max(case when topic = 'C' then value end) as topic_C
from Answers a
group by id_user, id_question;

顺便说一下，把它放回同一张桌子是不合理的.查询应该足够了.您也可以将其放入视图或另一个表中.

By the way, it wouldn't be reasonable to put this back into the same table. The query should be sufficient. You can also put this into a view or into another table.

另一种方法是使用连接.查询有点复杂:

An alternative approach is to use joins. The query is a bit more complicated:

select uq.id_user, uq.id_question, aa.value as topic_A,
       ab.value as topic_B, ac.value as topic_C
from (select distinct id_user, id_question from answers) uq left outer join
     Answers aa
     on aa.id_user = uq.id_user and aa.id_question = uq.id_question and
        aa.topic = 'A' left outer join
     Answers ab
     on ab.id_user = uq.id_user and ab.id_question = uq.id_question and
        ab.topic = 'B' left outer join
     Answers aa
     on ac.id_user = uq.id_user and ac.id_question = uq.id_question and
        ac.topic = 'C';

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