Mysql 如何构建SQL查询以生成三维矩阵？

我有两个mysql表（父表和子表）。我需要一个矩阵来显示垂直轴上的元素、水平轴上的月份以及交点上的值

要素： id，描述

眼病： id、月份、值、元素\u id

结果应该是这样的：

       |jan|feb|mar|apr|may|
orange | 12| 5 | 32| 12| 33|
apple  | 26| 2 | 45| 65| 59|
lemon  | 10| 9 | 11| 11| 96|
peanuts|  9| 5 | 59| 99| 87|
avocado|  7| 2 | 47|  3| 24|

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比如说：

select a.description, b.value as 'jan', c.value as 'feb', d.value as 'mar' from elements a,
(select * from ocurrences where month ='jan') b,
(select * from ocurrences where month ='feb') c,
(select * from ocurrences where month ='mar') d
where a.id = b.element_id 
and a.id = c.element_id 
and a.id = d.element_id 

架构（MySQL v5.7）

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查询#1

select a.description, b.value as 'jan', c.value as 'feb', d.value as 'mar' from elements a,
(select * from ocurrences where month ='jan') b,
(select * from ocurrences where month ='feb') c,
(select * from ocurrences where month ='mar') d
where a.id = b.element_id 
and a.id = c.element_id 
and a.id = d.element_id;

| description | jan | feb | mar |
| ----------- | --- | --- | --- |
| orange      | 12  | 5   | 32  |
| apple       | 26  | 2   | 45  |
| lemons      | 10  | 9   | 11  |

---

---

我建议条件聚合：

select e.description,
       max(case when o.month = 'jan' then o.value end) as jan,
       max(case when o.month = 'feb' then o.value end) as feb,
       max(case when o.month = 'mar' then o.value end) as mar,
       max(case when o.month = 'apr' then o.value end) as apr,
       max(case when o.month = 'may' then o.value end) as may
from elements e join
     occurrences o
     on e.id = o.id
group by e.description;

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条件聚合通常可以很好地实现这一点

假设“月”列包含“YYYYMM”格式的数字

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更新你的问题添加一个合适的数据样本实际上，听起来你想要一个数据透视表，所以你可以把每个月都放在它自己的列中。看到第三维度在哪里了吗？A在我看来是2D。我建议你研究一下应用程序代码

select e.description,
       max(case when o.month = 'jan' then o.value end) as jan,
       max(case when o.month = 'feb' then o.value end) as feb,
       max(case when o.month = 'mar' then o.value end) as mar,
       max(case when o.month = 'apr' then o.value end) as apr,
       max(case when o.month = 'may' then o.value end) as may
from elements e join
     occurrences o
     on e.id = o.id
group by e.description;

SELECT 
 elem.description,
 SUM(case when ocur.month = 201901 then ocur.value else 0 end) AS `jan`,
 SUM(case when ocur.month = 201902 then ocur.value else 0 end) AS `feb`,
 SUM(case when ocur.month = 201903 then ocur.value else 0 end) AS `mar`,
 SUM(case when ocur.month = 201904 then ocur.value else 0 end) AS `apr`,
 SUM(case when ocur.month = 201905 then ocur.value else 0 end) AS `may`
 -- Add for the other months here
FROM ocurrences ocur
LEFT JOIN elements elem ON elem.id = ocur.element_id
WHERE ocur.month BETWEEN 201901 AND 201912
GROUP BY ocur.element_id, elem.description
ORDER BY ocur.element_id