MERGE INTO
  mytable t
USING (
  SELECT
    :in_val AS val
  FROM
    rdb$database
) sel
ON (t.val = sel.val)
WHEN NOT MATCHED THEN
  INSERT (
    t.val
  ) VALUES (
    :in_val
  )
RETURNING
  NEW.id
INTO
  :var_res;

SELECT
  t.id
FROM mytable t
WHERE t.val = :in_val
INTO :var_res;

IF (:var_res IS NULL) THEN
  INSERT INTO mytable (val)
  VALUES (:in_val)
  RETURNING id
  INTO :var_res

SELECT
  t.id
FROM mytable t
WHERE t.val = :in_val
INTO :var_res;

IF (:var_res IS NULL) THEN
  MERGE INTO mytable t
  USING (
    SELECT
      :in_val AS val
    FROM rdb$database
  ) sel ON (t.val = sel.val)
  WHEN MATCHED THEN
    UPDATE SET
      t.val = :in_val
  WHEN NOT MATCHED THEN
    INSERT (
      t.val
    ) VALUES (
      :in_val
    )
  RETURNING
    t.id
  INTO
    :var_res;

WHEN SQLCODE -803 DO

CREATE TABLE mytable(
    ID    INTEGER NOT NULL GENERATED BY DEFAULT AS IDENTITY,
    VAL   INTEGER NOT NULL
);

ALTER TABLE mytable ADD CONSTRAINT pk_mytable PRIMARY KEY (id);
ALTER TABLE mytable ADD CONSTRAINT unq_mytable_val UNIQUE (val);

UPDATE OR INSERT INTO mytable (val)
VALUES (1)
MATCHING (val);

UPDATE OR INSERT INTO mytable (val)
VALUES (1)
MATCHING (val);

UPDATE OR INSERT INTO mytable (val)
VALUES (1)
MATCHING (val);