如何在 WHERE 中使用 AS 和别名？

Question

嗨，我想检查 NumOfSpeakLanguages 是否有超过 10 种语言，我该怎么做：这里的代码：

SELECT country.Name,country.Capital,COUNT(DISTINCT countrylanguage.Language) AS NumOfSpeakLanguages
FROM country,countrylanguage
WHERE country.Code=countrylanguage.CountryCode AND NumOfSpeakLanguages >10
GROUP BY country.Name

这里是错误代码：

SELECT country.Name,country.Capital,COUNT(DISTINCT countrylanguage.Language) AS NumOfSpeakLanguages
FROM country,countrylanguage
WHERE country.Code=countrylanguage.CountryCode AND NumOfSpeakLanguages >10
GROUP BY country.Name LIMIT 0, 25

#1054 - Unknown column'NumOfSpeakLanguages' in 'where clause'

谢谢！

Answer 1

对聚合函数条件使用 HAVING 子句：

SELECT country.Name,country.Capital,COUNT(DISTINCT countrylanguage.Language) AS NumOfSpeakLanguages
FROM country,
JOIN countrylanguage
  ON country.Code = countrylanguage.CountryCode
GROUP BY country.Name
HAVING COUNT(DISTINCT countrylanguage.Language) > 10
LIMIT 0, 25

@Andrew Brēza 评论道：你也可以说 HAVING NumOfSpeakLanguages >10。（这是一个 MySQL 特殊。）

也在做显式 JOIN!

或者，在派生的 table:

中进行聚合

select * from
(
    SELECT country.Name,country.Capital,COUNT(DISTINCT countrylanguage.Language) AS NumOfSpeakLanguages
    FROM country,
    JOIN countrylanguage
      ON country.Code = countrylanguage.CountryCode
    GROUP BY country.Name
) dt
where NumOfSpeakLanguages > 10
LIMIT 0, 25

如何在 WHERE 中使用 AS 和别名？

How can I use AS and the alias in WHERE?

mysql

sql

where-clause