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Projection and Existential Quantification - SQL
Intuitively it might seem that projection in SQL is simply a matter of specifying the required columns in the SELECT clause, as in Example.
Example: Projection (incorrect)
Student StudentId is enrolled on some course.
SELECT StudentId FROM IS_ENROLLED_ON
Unfortunately, though, if some student is currently enrolled on more than one course (as indeed student S1, Anne, is in our example database), then the row giving that student's id appears twice in the result, which because of that duplicate appearance does not represent a relation. To avoid multiple appearances of the same row SQL requires you to write the word DISTINCT. (The key word ALL can be given instead of DISTINCT, clarifying that duplicate rows are not to be eliminated. As ALL is the default option).
Effects of NULL in Aggregate Operator - SQL Let aggop(x) be an invocation of some aggregate operator aggop in SQL, where x is an expression (usually an open expression) to be
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