php - 获取相关表中的 SUM。如果没有找到，则求和为 0

Question

我有这个查询:

$sql = "SELECT m.id FROM members m
          LEFT JOIN orders o ON o.user_id = m.id AND
            YEAR(date) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH) AND
            MONTH(date) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)
          LEFT JOIN transactions t ON t.user_id = o.user_id";

我需要获取交易点数总和，所以我这样做:

$sql = "SELECT m.id, COALESCE(SUM(t.points), 0) AS total_points FROM members m
          LEFT JOIN orders o ON o.user_id = m.id AND
            YEAR(date) = YEAR(CURRENT_DATE - INTERVAL 1 MONTH) AND
            MONTH(date) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)
          LEFT JOIN transactions t ON t.user_id = o.user_id";

但是当我执行此操作时，我只会获得包含一些交易(和订单)的行。当没有找到交易时，我必须做什么才能获得 0 作为total_points？

文字描述:我想获取上个月创建的订单(订单表)的积分奖励总和(来自交易表)。

谢谢!

Answer 1

获取每个用户的积分总和，并在左连接中使用它，当用户没有交易时，得到0。

SELECT m.id, COALESCE(t.total_points, 0) AS total_points
FROM members m
LEFT JOIN orders o ON o.user_id = m.id AND
/* use this instead of functions which prevent indexes on the date column from being used */
o.date >= date '2016-11-01' AND o.date <= date '2016-11-30' 
LEFT JOIN (select user_id, SUM(points) total_points 
           from transactions group by user_id) t ON t.user_id = o.user_id