Postgresql中如何做百分比

Question

我試圖創建一個查詢。我有兩個表

• customer（customer_id）
• rental（rental_date（時間戳），customer_id）

SELECT to_char(rental_date, 'Month') AS month, 
     (count(Distinct customer.customer_id)/ 
      (count (distinct rental.customer_id) * 100) 
     ) AS percentage 
FROM RENTAL,customer 
GROUP BY month; 

但結果卻是零。

Answer 1

當一個操作涉及的所有「數字」都是整數時，結果也是一個整數。

試試這個方法：

SELECT 
    to_char(rental_date, 'Month') AS month, 

    (
    count(distinct customer.customer_id)::float/
    (count(distinct rental.customer_id)::float * 100::float) 
) 
    as percentage 

FROM 
    RENTAL,customer 
GROUP BY month;