validate last name is blank
SELECT
person.fullName,
(CASE WHEN 0 = CHARINDEX(' ', person.fullName)
then person.fullName
ELSE SUBSTRING(person.fullName, 1, CHARINDEX(' ', person.fullName)) end) as first_name,
(CASE WHEN 0 = CHARINDEX(' ', person.fullName)
THEN ''
ELSE SUBSTRING(person.fullName,CHARINDEX(' ', person.fullName), LEN(person.fullName) )end) last_name
FROM person