Using Aliased column in a join - sql

Question

I have an aliased colum:
FIRST_VALUE(SUBSTR(ba.CREATED,1,18))
OVER (PARTITION BY bsh.STRUCTURE_ELEMENT_ID, bal.BUDGET_CYCLE_ID
ORDER BY ba.CREATED DESC NULLS FIRST) AS UPDATED_DATE
I need to use this new field in a join, how can I do it? I've tried copying the same syntax but I get an error message stating window functions are not allowed here and also tried using the UPDATED_DATE alias name but that says the field does not exist.
Can anyone advise please?
Edited 24/10/15:
I've tried the suggestions I've been given but they don't seem to be working. I'm not sure if it's because it's already a complex statement with other joins in so this is the full code as it currently is
SELECT /* State Change 2 to 3 */
bal.BUDGET_CYCLE_ID AS BUDGET_CYCLE_ID,
bal.STRUCTURE_ELEMENT_ID AS COST_CENTRE,
FIRST_VALUE(SUBSTR(ba.CREATED,1,15))
OVER (PARTITION BY bsh.STRUCTURE_ELEMENT_ID,
bal.BUDGET_CYCLE_ID,
bsh.PREVIOUS_STATE || bsh.NEW_STATE
ORDER BY ba.CREATED DESC NULLS FIRST) AS UPDATED_DATE,
bsh.PREVIOUS_STATE AS PREVIOUS_STATE,
bsh.NEW_STATE AS NEW_STATE,
ba.USER_ID AS USER_ID
FROM BUDGET_ACTIVITY ba
LEFT JOIN BUDGET_ACTIVITY_LINK bal
ON ba.BUDGET_ACTIVITY_ID = bal.BUDGET_ACTIVITY_ID
AND ba.ACTIVITY_TYPE = 5
LEFT JOIN BUDGET_STATE_HISTORY bsh
ON bal.STRUCTURE_ELEMENT_ID = bsh.STRUCTURE_ELEMENT_ID
AND bal.BUDGET_CYCLE_ID = bsh.BUDGET_CYCLE_ID
AND SUBSTR(ba.CREATED,1,15) = SUBSTR(bsh.CHANGED_TIME,1,15)
WHERE PREVIOUS_STATE || NEW_STATE = 23
AND bal.budget_cycle_ID = '227565'
AND bal.structure_element_ID = '418'
I need to change the SUBSTR(ba.created,1,15) to the UPDATED_DATE field derived above. I'm relatively new to SQL and this one is beyond me.

Answer 1

You'll need to put your aliased column in a subquery:
SELECT
...
FROM
(
SELECT
...
FIRST_VALUE(SUBSTR(ba.CREATED,1,18))
OVER (PARTITION BY bsh.STRUCTURE_ELEMENT_ID, bal.BUDGET_CYCLE_ID
ORDER BY ba.CREATED DESC NULLS FIRST) AS UPDATED_DATE
FROM
...
) T1
JOIN TABLE2 T2 ON T1.UPDATED_DATE = T2.DATE_FIELD;