mysql IFNULL ELSE

and another way to skin that cat (flexible for not just null comparisons)...

select if(field_a is not null, field_a, field_b) from...

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+...

Yes, but it's just two parameters:

IFNULL(field_a,field_b)

If field_a is not null, it will be returned. Otherwise field_b will be returned.

Reference: IFNULL