MySQL的左联接不返回联接表中的行

Question

我有2个相同的表（_impressionsdaily，_impressionsalltime），在每天结束时，我将所有数据从每天移到所有时间。 结构为：ID，purchaseID，印象，唯一性，日期。

我运行以下查询：

SELECT  
    daily.purchaseID,
    SUM(daily.impression + IFNULL(alltime.impression, 0)) AS impression,
    SUM(daily.unique + IFNULL(alltime.unique, 0)) AS `unique`
    FROM _impressionsdaily daily
    LEFT JOIN _impressionsalltime alltime ON alltime.purchaseID= daily.purchaseID
    GROUP BY daily.purchaseID;

事实是，如果alltime表具有每天都不可用的购买，我将不会看到它们。

基本上，该想法是对每天和所有时间的所有印象进行汇总。

---编辑每天凌晨12:00，将每日表复制到所有位置，然后将其删除。 所以给出的解决方案是错误的。

感谢您的帮助，丹尼

Answer 1

我认为您正在寻找Union Select 。 尝试这样的事情：

SELECT DISTINCT(p.ID), p.purchaseID
FROM (SELECT *
      FROM _impressionsdaily
      UNION SELECT *
      FROM _impressionsalltime) AS p
GROUP BY p.purchaseID

Answer 2

SELECT  
daily.purchaseID,
SUM(daily.impression + IFNULL(alltime.impression, 0)) AS impression,
SUM(daily.unique + IFNULL(alltime.unique, 0)) AS `unique`
FROM _impressionsdaily daily
LEFT JOIN _impressionsalltime alltime ON alltime.purchaseID= daily.purchaseID
GROUP BY daily.purchaseID;

在这种情况下，您没有选择_impressionsalltime表的任何字段。

您也可以使用此sql从_impressionsalltime表中选择字段

SELECT  
alltime.*,
daily.purchaseID,
SUM(daily.impression + IFNULL(alltime.impression, 0)) AS impression,
SUM(daily.unique + IFNULL(alltime.unique, 0)) AS `unique`
FROM _impressionsdaily daily
LEFT JOIN _impressionsalltime alltime ON alltime.purchaseID= daily.purchaseID
GROUP BY daily.purchaseID;