如何將這些不同的列放入PostgreSQL的同一行？

Question

這裏是我的數據集（以哈希形式，因爲我使用的紅寶石寶石續集）;哈希名稱的列名：

{:item_category=>"Bacon", :year=>1890, :avg_yr_value=>0.12} 
{:item_category=>"Eggs (dozen)", :year=>1890, :avg_yr_value=>0.208} 
{:item_category=>"Bacon", :year=>1891, :avg_yr_value=>0.126} 
{:item_category=>"Eggs (dozen)", :year=>1891, :avg_yr_value=>0.221} 
{:item_category=>"Bacon", :year=>1892, :avg_yr_value=>0.129} 
{:item_category=>"Eggs (dozen)", :year=>1892, :avg_yr_value=>0.221} 
{:item_category=>"Bacon", :year=>1893, :avg_yr_value=>0.142} 
{:item_category=>"Eggs (dozen)", :year=>1893, :avg_yr_value=>0.224} 
{:item_category=>"Bacon", :year=>1894, :avg_yr_value=>0.185} 

這裏是我的代碼：

SELECT 
     string_agg(DISTINCT item_category, ' + ' ORDER BY item_category) AS items, 
     year, 
     sum(avg_yr_value) AS amount 

    FROM 
     earnings_and_prices 

    WHERE 
     item_category = 'Bacon' OR 
      item_category = 'Eggs (dozen)' 

    GROUP BY 
     year 

    HAVING 
     COUNT(DISTINCT item_category) = 2 

    ORDER BY 
     year 

我的結果（哈希值，因爲我使用了紅寶石的寶石續集）：

{:items=>"Bacon + Eggs (dozen)", :year=>1890, :amount=>0.328} 
{:items=>"Bacon + Eggs (dozen)", :year=>1891, :amount=>0.347} 
{:items=>"Bacon + Eggs (dozen)", :year=>1892, :amount=>0.35} 
{:items=>"Bacon + Eggs (dozen)", :year=>1893, :amount=>0.366} 

我想修改自己的代碼，以便它會包括：量「培根」和：量「雞蛋（打）」，類似這樣的，例如：

{:items=>"Bacon + Eggs (dozen)", :year=>1890, :amount=>0.328, :eggs_price=>0.208, :bacon_price=>0.12} 

我如何做到這一點？

Answer 1

對於具體情況，這個查詢適合：

SELECT 
    item_bacon || ' + ' || item_eggs AS items, 
    year, 
    bacon_avg_value + eggs_avg_value AS amount, 
    bacon_avg_value AS bacon_price, 
    eggs_avg_value AS eggs_price 
FROM (
    SELECT t1.item, t2.item, t1.year, t1.avg_value, t2.avg_value 
    FROM (
      SELECT item_category, year, avg_yr_value 
      FROM earnings_and_prices 
      WHERE item_category = 'Bacon' 
    ) AS t1(item, year, avg_value) 
    JOIN (
      SELECT item_category, year, avg_yr_value 
      FROM earnings_and_prices 
      WHERE item_category = 'Eggs (dozen)' 
    ) AS t2(item, year, avg_value) 
    USING(year) 
) AS t(item_bacon, item_eggs, year, bacon_avg_value, eggs_avg_value) 
ORDER BY year 

如果您需要更多的靈活性（兩個以上的項目進行篩選和展示），我會考慮使用動態SQL查詢。