Question

我获得了以下关系模式 -

country (countrycode,name,capital) &
population (populationcode,population,countrycode)

其中countrycode是外键w.r.t.关系国家。我必须使用最大人口来预测国家，但聚合操作MAX(population)不能直接用于给定的命令 - 。

SELECT pcode, ccode, name, capital
FROM population NATURAL JOIN country
WHERE ((SELECT MAX(population) as maxpopulation FROM population NATURAL JOIN country))

给定的数据集 -

INSERT INTO country VALUES(1,'INDIA','NEW DELHI');
INSERT INTO country VALUES(2,'U.S.A.','WASHINGTON DC');
INSERT INTO country VALUES(3,'U.K.','LONDON');
INSERT INTO country VALUES(4,'CHINA','BEIJING');
INSERT INTO country VALUES(5,'JAPAN','TOKYO');

INSERT INTO population VALUES(1,1200000000,1);
INSERT INTO population VALUES(2,350000000,2);
INSERT INTO population VALUES(3,65640000,3);
INSERT INTO population VALUES(4,1300000000,4);
INSERT INTO population VALUES(5,127000000,5);

因为它在终端中产生以下输出 -

+-------+-------+--------+---------------+
| pcode | ccode | name   | capital       |
+-------+-------+--------+---------------+
|     1 |     1 | INDIA  | NEW DELHI     |
|     2 |     2 | U.S.A. | WASHINGTON DC |
|     3 |     3 | U.K.   | LONDON        |
|     4 |     4 | CHINA  | BEIJING       |
|     5 |     5 | JAPAN  | TOKYO         |
+-------+-------+--------+---------------+
5 rows in set (0.00 sec)

预期的输出是 -

+-------+-------+--------+---------------+
| pcode | ccode | name   | capital       |
+-------+-------+--------+---------------+
|     4 |     4 | CHINA  | BEIJING       |
+-------+-------+--------+---------------+
1 rows in set (0.00 sec)

但它应该只告诉人口最多的国家属性是中国。

那么可以进行什么样的操作？

Answer 1

简单如下：

SELECT pcode, ccode, name, capital
FROM population NATURAL JOIN country
WHERE population = (SELECT MAX(population) FROM population)

Answer 2

请勿使用NATURAL JOIN。这是一种憎恶，因为它使用具有相同名称的键。它甚至不使用正确声明的外键关系。

如果您只想查找一行，那么我建议您ORDER BY和LIMIT：

select p.populationcode, p.countrycode, c.name, c.capital
from population p join
     country c
     using (countrycode)  -- of course, the traditional explicit "on" is totally correct too
order by p.population desc
limit 1;

当您使用比较最大人口的版本时，您可以获得重复项。如果这是你想要的，那么一定要使用那个版本。

Answer 3

在WHERE中使用子查询并使用LIMIT功能。

SELECT p.populationcode, c.countrycode, c.name, c.capital
FROM population p
INNER JOIN country c ON p.countrycode = c.countrycode
WHERE (SELECT countrycode 
       FROM (SELECT countrycode, population 
             FROM population 
             ORDER BY population DESC LIMIT 1)) = c.countrycode

在这种情况下如何进行多项选择操作？

3 个答案: