MySQL：带有 COUNT 和 GROUP BY 的查询中的 WHERE 子句？答案

【问题标题】：MySQL: WHERE-clause in query with COUNT and GROUP BY?MySQL：带有 COUNT 和 GROUP BY 的查询中的 WHERE 子句？
【发布时间】：2015-01-17 11:00:49
【问题描述】：

对于用于我的广告的每个类别，我想显示广告的总数。

我当前的查询如下所示：

SELECT
    categories_adverts.category,
    COUNT(adverts.id) AS numberOfAdverts
FROM adverts
LEFT JOIN categories_adverts
ON adverts.id_categories_adverts=categories_adverts.id
GROUP BY category ASC

我正在努力包含一个 WHERE 子句，该子句指定只显示那些持续时间仍然有效的广告。我试图在“FROM adverts”和“LEFT JOIN ...”之间加入以下子句：

WHERE TIME_TO_SEC(timediff(now(),adverts.stamp_created))<=adverts.duration

所以整个查询都是这样的，但这不起作用：

SELECT
    categories_adverts.category,
    COUNT(adverts.id) AS numberOfAdverts
FROM adverts
WHERE TIME_TO_SEC(timediff(now(),adverts.stamp_created))<=adverts.duration
LEFT JOIN categories_adverts
ON adverts.id_categories_adverts=categories_adverts.id
GROUP BY category ASC

查询有什么问题？

【问题讨论】：

标签： mysql sql

【解决方案1】：

你的 WHERE 子句应该在你的连接之后和分组和排序之前，像这样：

SELECT
    categories_adverts.category,
    COUNT(adverts.id) AS numberOfAdverts
FROM adverts
LEFT JOIN categories_adverts
ON adverts.id_categories_adverts=categories_adverts.id
WHERE TIME_TO_SEC(timediff(now(),adverts.stamp_created))<=adverts.duration
GROUP BY category ASC

【讨论】：

【解决方案2】：

where 子句位于joins 之后

SELECT c.category,
       COUNT(a.id) AS numberOfAdverts
FROM adverts a
LEFT JOIN categories_adverts c ON a.id_categories_adverts = c.id
WHERE TIME_TO_SEC(timediff(now(),a.stamp_created)) <= a.duration
GROUP BY c.category ASC

【讨论】：