Subquery in the Where clause

Question

Can I put a subquery in the where clause, on the left side on the operator?
This is a multi-row query.
Like this,
   select a.col1, a.col2, b.col1, b.col2,
             my_function(a.date1, b.date2) AS GROSSDAYS
   from table1 a
   where ( select ( a.date1 - b.date2 ) as range
               from     table1 a
               join      table2 b
               on       a.col3 = b.col3
               where rownum =1
               in ( 1,2,3)
and  a.col1 = b.col2I need to use a subquery because the column I need does not exist in the table, and I cannot make any changed to the table structure.
Is what I'm doing possible?
The subquery is the same as the function I have in the Select clause.

Answer 1

I tried a subquery in the where clause, but now I'm getting a missing expression error!
   SELECT
        r.complete_flag, r.order_num, r.referral_num, TRUNC(r.referral_datetime) AS referral_datetime,
        r.clinic_specialty, a.appointment_datetime, TRUNC(a.appointment_date_made) AS appt_datemade, a.appointment_status_id,
         scref.scr_rules.calcatcdays(r.referral_datetime,a.appointment_date_made) AS ATCDays  -- returns difference between two dates
FROM
        referral r,
                             ( SELECT referral_num,appointment_datetime, appointment_date_made, appointment_status_id
                                  FROM   ( SELECT referral_num, appointment_datetime, appointment_date_made, appointment_status_id,
                                            ROW_NUMBER() OVER (PARTITION BY referral_num
                                            ORDER BY appointment_datetime) rn
                                        FROM   appointment
                                 WHERE rn = 1
                         a
WHERE r.order_num IS NOT NULL
AND   ( SELECT adays                      -- THIS IS WHERE I'M GETTING A MISSING EXPRESSION ERROR!!!
       FROM ( SELECT scref.scr_rules.calcatcdays(r.referral_datetime,a.appointment_date_made) AS adays
                FROM referral r
                 JOIN appointment a
                 ON   a.referral_num = r.referral_num
                 WHERE ROWNUM = 1
       WHERE  adays
      ) = 3
AND   r.referral_num = a.referral_num(+)
AND   TRUNC(r.referral_datetime) >= TO_DATE('01-JUL-05','DD-MON-YY')
AND   TRUNC(r.referral_datetime) <= TO_DATE('31-JUL-05','DD-MON-YY')