Why we need to add all ORDER BY column names in SELECT clause when we are using DISTINCT in SELECT?

Question

Example :
Following first query execute but second prompt error.
1]
SELECT DISTINCT EId, Email, FirstName, LastName
FROM Employee
WHERE FirstName ='Yash' ORDER BY FirstName, LastName

2]
SELECT DISTINCT EId, Email, FirstName
FROM Employee
WHERE FirstName ='Yash' ORDER BY FirstName, LastName

Answer 1

because you don't select Last Name but you try to order by it in the second.
order by works on the "extracted" data, not on the original table (database), so if you did not include it in the Select it just does not exist for it.