need help w/ sql query - update and select at once

hi i'm trying to get this sql query to work, i'm trying to update and select at the same time, i'm not even sure if this is even possible update db.tablename set field1='yes' where (select * from db.tablename,db.tablename2 where db.tablename.userid=db.tablename2.userid); basically i have a column called 'userid'. this column exists in both tablename, and tablename2. where the value of userid in tablename is equal to the value of userid in tablename2, i'd like to set the value of field1 to 'yes'. can this be done from the command line? or am i better off writing another php page to accomplish this? thanks again redmond