如何在Postgres中連接行？

Question

我在Postgres中有一張預留表，我想要生成一個每月有一行的表格，並在列中顯示每年的收入（以及爲簡單起見，此處省略了很多其他內容）。

我可以通過硬編碼來做到這一點，但必須有更好的方法。我如何做到這一點擴展到x年？

謝謝！

CREATE TABLE reservations (
    checkin date NOT NULL, 
    earnings integer 
-- other data fields omitted 
); 

CREATE OR REPLACE FUNCTION compareYears() 
RETURNS TABLE(month double precision, earnings_2016 bigint, earnings_2017 bigint) 
AS 
$$ 
BEGIN 
RETURN QUERY 
with 
r2017 as (SELECT 
    date_part('month', reservations.checkin) AS month, 
    sum(reservations.earnings) as earnings_2017 
    FROM cd.reservations 
    WHERE date_part('year', reservations.checkin) = 2017 
    GROUP by date_part('month', reservations.checkin)), 
r2016 as (SELECT 
    date_part('month', reservations.checkin) AS month, 
    sum(reservations.earnings) as earnings_2016 
    FROM cd.reservations 
    WHERE date_part('year', reservations.checkin) = 2016 
    GROUP by date_part('month', reservations.checkin)) 
SELECT r2017.month, r2016.earnings_2016, r2017.earnings_2017 
FROM r2016, r2017 
WHERE r2017.month = r2016.month; 
END; 
$$ 
LANGUAGE 'plpgsql' VOLATILE; 

Answer 1

看看crosstab。我認爲它正是你需要的。 這是example