Projection in SQL - correct version

Student StudentId is enrolled on some course.

SELECT DISTINCT StudentId FROM IS_ENROLLED_ON

In more complicated examples it is sometimes quite difficult to tell whether omission of DISTINCT would give rise to duplicate rows. It might therefore seem good advice to always write DISTINCT. Indeed, I would certainly advocate such practice to students having to write SQL expressions in solutions to questions in exam papers, for example, but if it were followed blindly in commercial systems, then many queries would run very much slower than need be because typical SQL implementations have little or nothing in the way of built-in intelligence to recognize cases where duplicate rows cannot possibly arise. Although such intelligence is quite feasible within acceptable limits (and was used in Business System 12, for example), the inclusion of DISTINCT allows SQL implementations to place the responsibility for duplicate elimination on the user.