Question

我的询问的目标是返回所说的语言的国家，资本和数量。它还需要按递减的语言数量排序，然后按资本排序。最后，语言数量必须至少为5和10或更少。

这是我的问题：

SELECT country.name                    AS Country,
       city.name                       AS Capital,
       Count(countrylanguage.language) AS NumLanguages
FROM   country,
       city,
       countrylanguage
WHERE  city.id = country.capital
GROUP  BY city.name,
          country.name
HAVING ( Count(countrylanguage.language) BETWEEN 5 AND 10 );

它什么都不返回。必须使用where子句才能显示城市名称。在country表中只有一个id号，然后city表中包含id号和名称。

如果有人能发现我的错误，我将非常感激！

Answer 1

您错过了与countrylanguage的关系。没有它，你有一个笛卡尔积，所以Count(countrylanguage.language)等于countrylanguage中的记录数，最有可能大于10。

这是一个建议的解决方案（相应地调整字段名称/ DB结构）：

SELECT country.name                    AS Country,
       city.name                       AS Capital,
       Count(countrylanguage.language) AS NumLanguages
FROM   country,
       city,
       countrylanguage
WHERE  city.id = country.capital
AND    countrylanguage.language_id = country.language_id
GROUP  BY city.name,
          country.name
HAVING ( Count(countrylanguage.language) BETWEEN 5 AND 10 )
ORDER BY NumLanguages desc, city.Name

也就是说，您应该始终尝试避免查询的WHERE子句中的连接（隐式连接）。支持显式（声明性）连接将为您提供更多可读性和更大的灵活性。

<强>更新

根据评论建议，这里是使用ANSI-92连接语法的查询版本：

SELECT country.name                    AS Country,
       city.name                       AS Capital,
       Count(countrylanguage.language) AS NumLanguages
FROM       country
INNER JOIN city on city.id = country.capital
INNER JOIN countrylanguage on countrylanguage.language_id = country.language_id
GROUP  BY city.name,
          country.name
HAVING ( Count(countrylanguage.language) BETWEEN 5 AND 10 ); 
ORDER BY NumLanguages desc, city.Name

SQL查询不起作用，返回任何内容

1 个答案: