Use COALESCE:
SELECT COALESCE(field_a, field_b)
COALESCE is an ANSI standard function that returns the first non-null value from the list of columns specified, processing the columns from left to right. So in the example, if field_a is null, field_b value will be displayed. However, this function will return NULL if there is no non-null value from the columns specified.
It’s supported on MySQL (I’ve used it on 4.1), SQL Server (since v2000), Oracle 9i+…