T-SQL lacks a XOR logical operator, but it can be implemented from its definition:

A XOR B = (A AND NOT B) OR (NOT A AND B)

Sorry for the stupid example, I can't think of a better one right now: to find all users with NULL first_name (expression A) or NULL middle_name (expression B) but not both you could write:

-- This is how you would do it if T-SQL had a XOR operator.

SELECT *

FROM user

WHERE (first_name IS NULL) XOR (middle_name IS NULL)

-- This is how you have to code it with AND, OR and NOT operators

SELECT *

FROM user

WHERE (first_name IS NULL AND middle_name IS NOT NULL)

OR (first_name IS NOT NULL AND middle_name IS NULL)

Any boolean operator can be rewritten using AND, OR and NOT.