need help with xor

I'm trying to use XOR in an Access 2007 query and I'm not getting the behavior I expected.

Given a table "Foo" with an ID field and text field "SomeText"

where the recrods are:

ID SomeText
1 a
2 a
3
4

If I query the "SomeText" field with:

Like "a"

I get records 1 and 2

if I query the "SomeText" field with:

IS NULL

I get records 3 and 4

if I query the "SomeText" field with:

Like("a") xor IS NULL

I get records 1 and 2 (which is what I expect)

but... when I query the "SomeText" field with:

Like("b") xor IS NULL

I don't get ANY records. Shouldn't I get records 3 and 4?

Solution: need help with xor

I think the requirement is to show all rows containing "a" or all rows containing Null when no rows contain "a".

Select Foo.*
from Foo, (Select count(*) cnt from Foo where sometext = "a") as bar
where foo.sometext = "a" or (foo.sometext is NULL and bar.cnt = 0)