Oracle 18c

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CREATE TABLE metric (metrictid NUMBER PRIMARY KEY);
CREATE TABLE store (storeid NUMBER PRIMARY KEY);
CREATE TABLE idmapping (
  metricid REFERENCES metric,
  storeid  REFERENCES store
);
INSERT INTO IDMAPPING (metricid, storeid)
SELECT m.metrictid, s.storeid
FROM   METRIC m
       CROSS JOIN store s
WHERE  m.metrictid = 50441
AND    s.storeid = 18198;
DECLARE
  parent_key_not_found EXCEPTION;
  PRAGMA EXCEPTION_INIT(parent_key_not_found, -2291);
BEGIN
  INSERT INTO idmapping (metricid, storeid) VALUES (50441, 18198);
EXCEPTION
  WHEN parent_key_not_found THEN
    -- Ignore the exception
    NULL;
END;
/
1 rows affected