Discussion
Which of the following SQL statements are helpful in database redesign?
Answer: C
Correct answer: Both of the above are helpful
Both belong to the same family and both are used in redesign. A correlated subquery is evaluated once for each candidate row of the outer query, which is what lets it compare a row against others in the same table - the way a functional dependency is tested. EXISTS and NOT EXISTS are the standard form such a subquery takes, since they ask only whether any matching row exists rather than what it contains.
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