How can I (safely) set a table column to be a unique id number? I'd just like to set all the columns that satisfy this WHERE clause to be one more than the MAX value of the column. Like so: UPDATE sample SET status='CONFIRMING', orderid=MAX(orderid)+1 WHERE customerid='2' AND orderid='-1'; MySQL gives me 'ERROR 1111: Invalid use of group function'. I assume I can't use MAX(orderid)+1 in this context, but I don't want to do this in 2 sql statements, because I just might (however small the chance) get same same orderid on 2+ orders ... (pending orders always have a orderid of '-1') make sense? James.