sposob pre MS SQL:

DECLARE @ID INT, @NEW_ID
DECLARE XCUR CURSOR FOR
SELECT ID FROM DOKLADY

OPEN XCUR

FETCH NEXT FROM XCUR INTO @ID

WHILE @@FETCH_STATUS = 0
BEGIN
  INSERT INTO DOKLADY_SYSTEM2 (POLIA)
  SELECT POLIA
  FROM DOKLADY
  WHERE ID = @ID

  SET @NEW_ID = SCOPE_IDENTITY()  -- taktiez mozes pouzit @@IDENTITY

  INSERT INTO POLOZKY_SYSTEM2 (DOKLAD_ID, POLIA)
  SELECT @NEW_ID, POLIA
  FROM POLOZKY
  WHERE DOKLAD_DID = @ID

  FETCH NEXT FROM XCUR INTO @ID
END

CLOSE XCUR

DEALLOCATE XCUR

samozrejme vacsina systemov ma okrem automatickeho cislovania aj vygenerovane cislo dokladu, v pripade, ze by si chcel okrem vkladania z jedneho systemu do druheho robit aj synchronizaciu dokladov. Vazba je vsak uplne normalne cez primary key.