ruby - 如何在这个 PostgreSQL 数据集中找到每年两个类别的总和？

Question

这是我的数据集（CSV 格式，便于阅读）：

item_category,year,price
"Bacon",1890,0.12
"Bacon",1891,0.126
"Bacon",1892,0.129
"Bacon",1893,0.142
"Eggs (dozen)",1890,0.208
"Eggs (dozen)",1891,0.221
"Eggs (dozen)",1892,0.221
"Eggs (dozen)",1893,0.224

我想找出每年培根和鸡蛋的价格总和。所以，我想要的结果是：

item_category,year,price
"Bacon and eggs",1890,0.328
"Bacon and eggs",1891,0.347
"Bacon and eggs",1892,0.35
"Bacon and eggs",1893,0.366

我是 SQL 的新手，我一直在研究 Self-Join 方法，但它们似乎并不是我正在寻找的——我现在基本上已经没有想法了。

如果有帮助，我还知道如何使用 Sequel gem for Ruby。

Answer 1

您似乎正在寻找GROUP BY和聚合函数，而不是自联接：

SELECT string_agg(DISTINCT item_category, ' and ' ORDER BY item_category)
                                                                   AS items
     , year
     , sum(price) AS price
FROM   tbl
GROUP  BY year
ORDER  BY year

string_agg(DISTINCT item_category, ' and ' ORDER BY item_category)可选择连接不同的名称以自动形成新标签。可以为你工作。否则用您的静态标签替换它。

Answer 2

select "Bacon and eggs", year, sum(price) 
from    table_name 
group by "Bacon and eggs", year
order by year;