Postgres 13

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CREATE TABLE t1 (id int);
INSERT INTO t1 VALUES (1),(3),(5);

CREATE TABLE t2 (id int2);
INSERT INTO t2 VALUES (1),(2),(3),(4),(5);

CREATE TABLE

INSERT 0 3

CREATE TABLE

INSERT 0 5

INSERT INTO t1 (id)
SELECT t2.id FROM t2
WHERE NOT EXISTS (
  SELECT * FROM t1
  WHERE t2.id = t1.id);

INSERT 0 2

SELECT * FROM t1 ORDER BY id;

id

1

2

3

4

5

SELECT 5