Alias Not Usable In WHERE Part?

why this query works fine:

SELECT Z.ZAJEZD_ID, Z.JMENO,
(
SELECT COUNT(ZAJEZD_ID)
FROM trasa T
WHERE T.ZAJEZD_ID = Z.ZAJEZD_ID
) AS POCET_ZASTAVEK
FROM zajezd Z
ORDER BY Z.JMENO

But when I try to use the alias to filter the results like this (note the WHERE part):

SELECT Z.ZAJEZD_ID, Z.JMENO,
(
SELECT COUNT(ZAJEZD_ID)
FROM trasa T
WHERE T.ZAJEZD_ID = Z.ZAJEZD_ID
) AS POCET_ZASTAVEK

[code]...

Then it throws an error message ORA-00904: Unknown Identifier for the column/alias POCET_ZASTAVEK?