This sample demonstrates use of subquery in SET clause.
The query works as follows. First, only those entries are picked that have "Ticket_price" 5 of less. Then corresponding venue is taken, its capacity is multiplied by two and set as the new ticket price. Notice that NULL fields are not affected.
update Events e
set Ticket_price = (select Capacity from Venues where VenueNo=e.VenueNo)*2
where Ticket_price <= 5