LifeinAsia - 3:18 pm on Apr 7, 2010 (gmt 0)
why not ...
Because that would set the value for EVERY column1 (when blank or null), regardless of the value of column2. If the OP only wants to change the rows where column2 equals value1, but not where column2 equals some other value, code will do what is needed. For changing EVERY column1, your code is the correct method.
Reading the OP again, I see that I focused on the query, not the text explanation. Good catch!