Default Column Value

[Bill Guion]

In one of my tables I have a start_date - timestamp (2007-07-04). A 
separate column, start_yr_mo, has 200704 (first seven characters of 
timestamp without the '-'). Both are entered manually. Can I define 
start_yr_mo as a default of, for example, set start_yr_mo = 
concat(substr(start_date, 1, 4),substr(start_date,6,2)). Something 
like

ALTER TABLE events
ALTER start_yr_mo start_yr_mo
SET = concat(substr(start_date, 1, 4),substr(start_date,6,2));

      -----===== Bill =====-----