Updating Multiple Rows with Data from Column A into Column B? I have about 60,000 rows that I'm needing to update the information from column_a to column_b.  For example, 60,000 customers need their arrival_time to match their departure_time.  I understand that I can do this with one row and it work with a nested select statement in an update statement.  When trying to update multiple rows though I believe I'm getting stuck on having to have 60,000 unique key identifiers.  

This was what I used to update 1 of the records, I'm using Oracle SQL:

UPDATE patient 
SET discharge_dt = (SELECT admit_dt 
                    FROM patient 
                    WHERE pat_seq = 'XXXXXX') 
WHERE facility_id = 'X' 
    AND pat_seq = 'XXXXXX'


Apologize for the confusing description, not really a database administrator myself. 

Any help on this would be greatly appreciated.
If you just want the discharge_dt to be overwritten with the admit_dt from the same row, it sounds like you just need

UPDATE patient
   SET discharge_dt = admit_dt
 WHERE facility_id = 'X'
   AND pat_seq IN (&lt;&lt;query that returns the 60,000 keys&gt;&gt;)