Question

首先，微小的查询工作，但我需要在它的中间放置一些连接来创建一个条件。微小的查询如下：

SELECT COUNT(vis.id), country.cod
FROM visits AS vis, countries AS country
WHERE country.description = vis.country
GROUP BY country.cod

现在的问题是我需要在where子句中添加另一个条件，并且这个条件必然来自两个表中的连接。

SELECT COUNT(vis.id), country.cod
FROM visits AS vis, countries AS country
INNER JOIN products AS prod ON prod.id = vis.id_product
INNER JOIN customers AS cust ON cust.id = prod.id_customer
WHERE country.description = visit.country AND prod.id_customer = 13
GROUP BY country.cod

我收到的错误是：

#1054 - Unknown column 'vis.id_product' in 'on clause'

相信我，那个领域存在。此外，我已经尝试过，只是为了测试，将其他字段替换为id_product而错误是相同的。

Answer 1

您已为表visits提供了别名vis。现在你需要使用它。

SELECT COUNT(vis.id), country.cod
FROM visits AS vis JOIN
     countries AS country
     ON  country.description = vis.country JOIN
     products AS prod
     ON prod.id = vis.id_product JOIN
     customers AS cust
     ON cust.id = prod.id_customer
WHERE prod.id_customer = 13
GROUP BY country.cod;

此外，您正在混合隐式和显式连接。仅使用显式连接，其中连接条件位于on子句中，而不是where子句中。一个简单的规则：永远不要在from子句中使用逗号。

SQL Query无法使用连接

1 个答案: