SQL子查询和列命名问题

Question

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1. 这是正确的代码

   SELECT MAX(inflation_rate) AS max_inf
     FROM (
         SELECT name, continent, inflation_rate
         FROM countries
         INNER JOIN economies
         USING (code)
         WHERE year = 2015) AS subquery
   GROUP BY continent;
   

2. 这是错误的代码

   SELECT MAX(subquery.economies.inflation_rate) AS max_inf
   FROM
   (SELECT countries.name, countries.continent, economies.inflation_rate
   
   FROM countries
   
   INNER JOIN economies
   
   ON countries.code = economies.code
   
   WHERE economies.year = 2015) AS subquery
   GROUP BY subquery.countries.continent;
   

为什么不允许第二个？

Answer 1

SELECT 
    MAX(subquery.economies.inflation_rate) AS max_inf   -- 3
FROM (
    SELECT 
        countries.name,                                 -- 1 
        countries.continent, 
        economies.inflation_rate
    FROM ...) AS subquery                               -- 2
GROUP BY 
    subquery.countries.continent;                       -- 3

您正在使用子查询 (2)。此子查询返回三列:名称、大陆、inflation_rate (1)。只有这些名称在子查询之外是已知的，而其他的则一无所知。因此，上级查询不知道列名从何而来。表或表模式是不相关的。

因此，对于上级查询，唯一相关的信息是:子查询的名称和列名称 (3):

SELECT 
    MAX(subquery.inflation_rate) AS max_inf   -- change
FROM (
    SELECT 
        countries.name,                                 
        countries.continent, 
        economies.inflation_rate
    FROM ...) AS subquery                               
GROUP BY 
    subquery.continent;                       -- change