连接垂直和水平表答案

【问题标题】：Joining vertical and horizontal table连接垂直和水平表
【发布时间】：2022-01-24 20:57:03
【问题描述】：

我怎样才能做一个连接两个表并获得结果表。考虑它有困难，因为我相信一个是水平表，另一个是垂直表。关于 SO 的其他答案对我来说并不清楚，因为我必须在一行中加入一个带有列名的值。我该怎么做？

CREATE TABLE forecast (
   year integer,
   week integer,
   model varchar(50),
   category varchar(50),
   subcategory varchar(50)
);  
insert into forecast (year, week, model, category, subcategory) values (2021, 1, 'AAA', 'CategoryA', 'SubcategoryA');
insert into forecast (year, week, model, category, subcategory) values (2021, 1, 'BBB', 'CategoryA', 'SubcategoryA');
insert into forecast (year, week, model, category, subcategory) values (2021, 1, 'CCC', 'CategoryB', 'SubcategoryB');
insert into forecast (year, week, model, category, subcategory) values (2021, 1, 'DDD', 'CategoryA', 'SubcategoryC');

CREATE TABLE translation (
    type varchar(50),
    name varchar(50),
    translated varchar(50)
); 
insert into translation (type, name, translated) values ('category', 'CategoryA', 'TranslatedCategoryA');
insert into translation (type, name, translated) values ('category', 'CategoryB', 'TranslatedCategoryB');
insert into translation (type, name, translated) values ('subcategory', 'SubcategoryA', 'TranslatedSubcategoryA');
insert into translation (type, name, translated) values ('subcategory', 'SubcategoryB', 'TranslatedSubcategoryB');
insert into translation (type, name, translated) values ('subcategory', 'SubcategoryC', 'TranslatedSubcategoryC');



CREATE TABLE result (
    year integer,
    week integer,
    model varchar(50),
    category varchar(50),
    subcategory varchar(50)
); 


insert into result (year, week, model, category, subcategory) values (2021, 1, 'AAA', 'TranslatedCategoryA', 'TranslatedSubcategoryA');
insert into result (year, week, model, category, subcategory) values (2021, 1, 'BBB', 'TranslatedCategoryA', 'TranslatedSubcategoryA');
insert into result (year, week, model, category, subcategory) values (2021, 1, 'CCC', 'TranslatedCategoryB', 'TranslatedSubcategoryB');
insert into result (year, week, model, category, subcategory) values (2021, 1, 'DDD', 'TranslatedCategoryA', 'TranslatedSubcategoryC');

这个

select * from forecast f
left join translation t
on t.name = f.category or t.name = f.subcategory

一次翻译一个，这是有道理的，但我无法从中得到两列来翻译每一列

【问题讨论】：

标签： sql tsql join

【解决方案1】：

类型和名称的双左连接。
如果没有翻译，则合并默认为原始名称。

select f.year, f.week, f.model
, coalesce(cat.translated, f.category) as category 
, coalesce(subcat.translated, f.subcategory) as subcategory
from forecast f
left join translation cat 
  on cat.name = f.category 
 and cat.type = 'category' 
left join translation subcat 
  on subcat.name = f.subcategory
 and subcat.type = 'subcategory'
order by f.year, f.week, f.model;

year	week	model	category	subcategory
2021	1	AAA	TranslatedCategoryA	TranslatedSubcategoryA
2021	1	BBB	TranslatedCategoryA	TranslatedSubcategoryA
2021	1	CCC	TranslatedCategoryB	TranslatedSubcategoryB
2021	1	DDD	TranslatedCategoryA	TranslatedSubcategoryC

db小提琴here

【讨论】：

【解决方案2】：

我们必须进行多重连接。每列一个。

select f.year, f.week, f.model, t.translated as 'category', t2.translated as 'subcategory' 
from forecast f
left join translation t
on t.name = f.category 
left join translation t2
on t2.name = f.subcategory

【讨论】：

旁白：一个好习惯，也是一个值得鼓励的习惯，是分配至少有微弱意义的表别名。 t 和 t2 交流不多。即使tc 和tsc 也会有所改进。