Using same column from one table with two foreign keys on other table

Question

I have to do an exercise on SQL, i ve got two tables:

create table REPRESENTA (
cod_pais1 varchar (5),
cod_pais2 varchar (5),
primary key(cod_pais1,cod_pais2));
insert into representa values('p002','p003');
insert into representa values('p001','p004');

create table PAIS(
Cod_pais varchar (5)primary key unique,
nombre varchar (25) not null,
continente varchar (25)not null,
clubes numeric (9));
insert into pais values('p001','rusia','europa','10');
insert into pais values('p002','francia','europa','3');
insert into pais values('p003','guayana francesa','america','1');
insert into pais values('p004','uzbekistan','asia','8');
insert into pais values('p005','nigeria','africa','14');

I have to make a consult to show REPRESENTA tables as it is but with the names related from PAIS table. And the professor doesnt allow use subselects.

I tried with

select pais.nombre as A, pais.nombre as B from pais join representa R  on          
pais.Cod_pais=representa.cod_pais1 join representa on  
pais.Cod_pais=  representa.cod_pais2;

but it goes error if i use the same table twice.

Answer 1

I think you are looking for something like below.

select p1.nombre as a, p2.nombre as b
from representa R
join PAIS p1
on p1.Cod_pais = R.cod_pais1
join PAIS p2
on p2.cod_pais = R.cod_pais2

The idea here is if you want to show nombre associted with one another in table representa , you need to join table PAIS twice to table representa ; 1 time for nombre associated with cod_pais1 and 2nd time for nombre associated with cod_pais2 .

Result:

+---------+------------------+
|    a    |        b         |
+---------+------------------+
| francia | guayana francesa |
| rusia   | uzbekistan       |
+---------+------------------+

DEMO