What is the output of the following qury?
SELECT EnrollmentDate, COUNT(*) AS StudentCount
FROM Person
WHERE EnrollmentDate IS NOT NULL

When the above query is run, you get the following error:
Column 'Person.EnrollmentDate' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Because it requires a groupBy or an Aggregate function clause as you are performing Count.