Question

我有两张桌子：

COUNTRY (land_name, land_code)
CONTINENT (land_code, cont)

我需要创建两个查询：

获取位于两大洲的所有国家（假设这是唯一的可能性，没有超过2个大陆的国家）。输出表单（land_name, Cont1, Cont2）我试图创建一个查询，至少从CONTINENT表中获取值：
```
SELECT land_code, MAX (cont) AS "K1", MIN(cont) AS "K2"
FROM CONTINENT 
GROUP BY land_code;
```
但是在输出中我得到了所有国家的名单，即使他们只在一个大陆上
获取位于同一大洲的国家/地区（land_name1, land_name2, Cont1, Cont2）

COUNTRIES (land_name, land_code, capital)
Austria A Vienna
Turkey TR Ankara
Russia RU Moscow
Italy I Rome
France F Paris
Egypt ET Cairo

CONTINENTS (land_code, Continent, Percentage)
ET Africa 90
ET Asia 10
F Europe 100
RU Asia 80
RU Europe 20
TR Asia 68
TR Europe 32

Answer 1

1假设没有超过2个大陆的国家/地区，您可以使用此查询：

select land_name, cont1, cont2
from (select c.land_name, min(t.cont) cont1, max(t.cont) cont2
      from country c join continent t on c.land_code = t.land_code group by c.land_name) t
 where t.cont1 != t.cont2;

2同一大洲的所有国家/地区：

select distinct t1.land_name, t2.land_name, c.cont
from  country t1
      join country t2 on t1.land_name < t2.land_name
      join continent c on c.land_code = t1.land_code
 where t1.land_code = t2.land_code;

如果你需要展示两个大陆共同的国家，你可以试试这个：

select t1.land_name, t2.land_name, t1.cont1, t1.cont2
from (select c.land_name, min(t.cont) cont1, max(t.cont) cont2
      from country c join continent t 
           on c.land_code = t.land_code 
      group by c.land_name) t1
      join
     (select c.land_name, min(t.cont) cont1, max(t.cont) cont2
      from country c join continent t 
           on c.land_code = t.land_code 
      group by c.land_name) t2 
      on t1.land_name < t2.land_name     
 where t1.cont1 != t1.cont2
   and t1.cont1 = t2.cont1 and t1.cont2 = t2.cont2;

连接具有相似字段的行

1 个答案: