SQL：从表1分的结果多行合并2行至一个子查询

Question

我有什么：

表具有两列 水果 食品（我从中选择水果）

一每个水果，一个小尺寸，一个大尺寸。

id | category | subcategory | title | description  | value 
----------------------------------------------------------------- 
1 | Fruit | Small  | Plum | Om, nom, nom!  | 0.50 
2 | Fruit | Large  | Plum | Om, nom, nom!  | 1.50 
3 | Fruit | Small  | Orange | Mmm, citrus.  | 0.30 
4 | Fruit | Large  | Orange | Mmm, citrus.  | 0.75 
5 | Fruit | Small  | Melon | Get in mah belly! | 2.00 
6 | Fruit | Large  | Melon | Get in mah belly! | 3.10 

我需要什么：

我需要两行对每个水果组合成一排：

类，冠军和描述将始终是相同的为每一对行。

ID，子类别和值将永远为每对行的不同。

id.r1 | id.r2 | category.r1 | title.r1 | description.r1  | subcategory.r1 | value.r1 | subcategory.r2 | value.r2 
----------------------------------------------------------------------------------------------------------------------- 
1  | 2  | Fruit  | Plum  | Om, nom, nom!  | Small   | 0.50  | Large   | 1.50 
3  | 4  | Fruit  | Orange | Mmm, citrus.   | Small   | 0.30  | Large   | 0.75 
5  | 6  | Fruit  | Melon  | Get in mah belly! | Small   | 2.00  | Large   | 3.10 

我已经试过什么：

SELECT r1.id, 
(SELECT r2.id FROM `my_table` r2 WHERE r1.title = r2.title), 
r1.category, 
r1.subcategory, 
(SELECT r2.category FROM `my_table` r2 WHERE r1.title = r2.title) 
r1.title, 
r1.description, 
r1.value, 
(SELECT r2.value FROM `my_table` r2 WHERE r1.title = r2.title) 

FROM `my_table` r1 

WHERE category = "Fruit" 

...主要生产：

子查询返回多个1行

我的问题：

如何修改上述查询以实现我所描述的内容？

Answer 1

你可以很容易地做到这一点与聚合：

select category, title, description, 
     MAX(case when subcategory = 'Small' then id end) as Small_Id, 
     MAX(case when subcategory = 'Small' then value end) as Small_Value, 
     MAX(case when subcategory = 'Large' then id end) as Large_Id, 
     MAX(case when subcategory = 'Large' then value end) as Large_Value 
from my_table f 
group by category, title, description 

注：这不包括在最终结果subcategory因为该行没有子类别。

你也可以做到这一点的加盟，这似乎是你正在采取的路径：

select fsmall.id, flarge.id, fsmall.category, flarge.category, . . . 
from my_table fsmall join 
    my_table flarge 
    on fsmall.subcategory = 'small' and flarge.subcategory = 'large' and 
     fsmall.category = flarge.category and 
     fsmall.title = flarge.title and 
     fsmall.description = flarge.description 

根据是否可能有一些失踪行，你可能需要一个left outer join或full outer join。