MySQL：查詢中的WHERE子句與COUNT和GROUP BY？

Question

對於用於我的廣告的每個類別，我想顯示廣告的總數。

我當前的查詢看起來是這樣的：

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 

我與包括其specificies，只有那些廣告中顯示，其持續時間仍是有效的WHERE子句中掙扎。我想有間「FROM廣告」和下列條款「LEFT JOIN ...」：

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

所以整個查詢想這樣的，但是，這並不工作：

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 

查詢有什麼問題？

Answer 1

你的WHERE子句應該是你加入之前分組和排序， 這樣後：

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 

Answer 2

的where子句來後join小號

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