sql left join

두 개 테이블을 하나로 취합하는 union .. 하찮은 left join 예시가 있어요.

https://www.db-fiddle.com/f/oEsktRp2KnsTHtVWN6y8Yk/0

CREATE TABLE seq(
  pk0 INTEGER NOT NULL PRIMARY KEY
);
CREATE TABLE test1(
  pk1 INTEGER NOT NULL PRIMARY KEY,
  pkv1 VARCHAR(64) NOT NULL
);
CREATE TABLE test2(
  pk2 INTEGER NOT NULL PRIMARY KEY,
  pkv2 VARCHAR(64) NOT NULL
);

INSERT INTO seq(pk0)VALUES(1),(2),(3),(4),(5),(6);
INSERT INTO test1(pk1,pkv1)VALUES(1,"a"),(2,"b"),(3,"c");
INSERT INTO test2(pk2,pkv2)VALUES(4,"d"),(5,"e"),(6,"f");

SELECT * FROM seq
	LEFT JOIN test1 ON pk1=pk0
    LEFT JOIN test2 ON pk2=pk0;

SELECT pk0, COALESCE(pkv1,pkv2) pkv0
	FROM seq
	LEFT JOIN test1 ON pk1=pk0
    LEFT JOIN test2 ON pk2=pk0;