Updating work hours

Question

Greetings everyone,
I'm trying to update a field in a table with the total work hours in a specific month. To achieve this I've tried using a piece of SQL found in several other threads to retrieve the number of work days in a month. This doesn't work properly in my case however, since the subquery doesn't know the reference to a field on a higher level. Here is the SQL:
UPDATE PS_EMPL_WRK_HRS Z SET WRK_HRS = (
SELECT Z.STD_HOURS * (SELECT COUNT(*)
    from (select trunc(Z.EFFDT, 'mm')+level-1 dt
           FROM   DUAL
           connect by level <= last_day(Z.EFFDT) - trunc(Z.EFFDT, 'mm')+1
    where to_char(dt, 'dy' ) not in ('sat', 'sun'))  END AS WRK_HOURS
FROM PS_GENL_DEDUCT_NLD GD
WHERE   Z.EMPLID = GD.EMPLID
  AND Z.EMPL_RCD = GD.EMPL_RCD
  AND (GD.DEDUCTION_END_DT >= Z.EFFDT OR GD.DEDUCTION_END_DT IS NULL)
)The error message will be ORA-00904: "Z"."EFFDT": invalid identifier, because the subquery to retrieve the number of workdays in a month doesn't know about the Z alias.
The PS_EMPL_WRK_HRS contains the EMPLID, EFFDT and STD_HOURS. For each employee the record contains one row for each last day of the month, which is stored in the EFFDT field.
Any help would be appreciated!
CREATE TABLE PS_EMPL_WRK_HRS (EMPLID VARCHAR2(11) NOT NULL,
   EFFDT DATE,
   STD_HOURS DECIMAL(6, 2) NOT NULL,
   WRK_HOURS DECIMAL(6, 2)
Insert into PS_EMPL_WRK_HRS (EMPLID,EFFDT,STD_HOURS) values ('010382',to_date('31-05-2013','DD-MM-RRRR'),40);
Insert into PS_EMPL_WRK_HRS (EMPLID,EFFDT,STD_HOURS) values ('010382',to_date('30-04-2013','DD-MM-RRRR'),40);
Insert into PS_EMPL_WRK_HRS (EMPLID,EFFDT,STD_HOURS) values ('010382',to_date('31-03-2013','DD-MM-RRRR'),40);
Insert into PS_EMPL_WRK_HRS (EMPLID,EFFDT,STD_HOURS) values ('010382',to_date('28-02-2013','DD-MM-RRRR'),40);
Insert into PS_EMPL_WRK_HRS (EMPLID,EFFDT,STD_HOURS) values ('010382',to_date('31-01-2013','DD-MM-RRRR'),40);Edited by: Michel on Feb 5, 2013 3:53 PM - Added CREATE and INSERT script

Answer 1

Fixed it already:
UPDATE PS_EMPL_WRK_HRS Z SET  WRK_HOURS  = (
CASE WHEN (SELECT
1
FROM PS_GENL_DEDUCT_NLD GD
WHERE   Z.EMPLID = GD.EMPLID
  AND Z.EMPL_RCD = GD.EMPL_RCD
  AND (GD.DEDUCTION_END_DT >= Z.EFFDT OR GD.DEDUCTION_END_DT IS NULL)) = 1
THEN 1 ELSE (Z.STD_HOURS - Z.DED_HRS_ILL_K ) * (SELECT COUNT(*)
        FROM
          (SELECT rownum rnum
          FROM ps_country_tbl
          ) Y
        WHERE y.rnum <= Z.EFFDT - to_date(TO_CHAR(Z.EFFDT,'YYYY-MM')||'01','YYYY-MM-DD') + 1
        AND TO_CHAR( Z.EFFDT + Y.RNUM - 1, 'D' ) NOT IN ( '1', '7' )
        ) END );