sql server 2008 - Compare and replace values in MS Access -

i want match between 2 fields in same table. if value null , value b not null, replace value value value b. if value not null , value b null, no change table.......？ want compare whole column of data.

you can use column names both l-values , r-values in update statement:

update mytable set    b = a, = null  not null , b null; 

sqlfiddle