Starting with Sql Server 2012: string concatenation function CONCAT
converts to string implicitly. Therefore, another option is
SELECT Id AS 'PatientId',
CONCAT(ParentId,'') AS 'ParentId'
FROM Patients
CONCAT
converts null
values to empty strings.
Some will consider this hacky, because it merely exploits a side effect of a function while the function itself isn't required for the task in hand.