Oracle 18c

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CREATE TABLE borders(
  Country1 VARCHAR(4),
  Country2 VARCHAR(4),
  Length DECIMAL 
  CHECK (Length > 0),
  CONSTRAINT BorderKey PRIMARY KEY (Country1,Country2) 
);
INSERT INTO borders VALUES('A1','A2',1000)
1 rows affected

CREATE TABLE Economy(
  Country VARCHAR(4) CONSTRAINT EconomyKey PRIMARY KEY,
  GDP DECIMAL CONSTRAINT EconomyGDP
  CHECK (GDP >= 0)
);
INSERT INTO Economy VALUES ('A1',100);
1 rows affected

INSERT INTO Economy VALUES('A2',101);
1 rows affected

SELECT b.Country1,b.Country2, e1.GDP as GDP1,e2.GDP AS GDP2,b.Length
FROM borders b 
JOIN Economy e1 ON b.Country1 = e1.Country
JOIN Economy e2 ON b.Country2 = e2.Country


COUNTRY1
COUNTRY2
GDP1
GDP2
LENGTH


A1
A2
100
101
1000