Problem solve Get help with specific problems with your technologies, process and projects. This is the same thing as finding two equal sets. How many ways can you find to do this problem? Printed with permission from Morgan Kaufmann, a division of Elsevier.
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Problem solve Get help with specific problems with your technologies, process and projects. This is the same thing as finding two equal sets. How many ways can you find to do this problem? Printed with permission from Morgan Kaufmann, a division of Elsevier. Copyright For more information about this book, and other similar titles, please visit www. Set theory has two symbols for subsets. One is a "horseshoe" on its side, which means that set A is contained within set B and is sometimes called a proper subset.
The other is the same symbol with a horizontal bar under it, which means "contained in or equal to," which is sometimes called just a subset or containment operator. Standard SQL has never had an operator to compare tables against each other. The IN predicate is a test for membership, not for subsets. Membership is for one element, while a subset is itself a set, not just an element.
This tells you which pairs are not matched, not who is. The final step is to remove these nonmatching pairs from all possible pairs. Answer 2 The usual way of proving that two sets are equal to each other is to show that set A contains set B, and set B contains set A.
What you would usually do in standard SQL would be to show that there exists no element in set A that is not in set B, and therefore A is a subset of B. First, I join one supplier to another on their common parts, eliminating the situation where supplier 1 is the same as supplier 2, so that I have the intersection of the two sets.
If the intersection has the same number of pairs as each of the two sets has elements, then the two sets are equal. You have to be careful about using the ALL clauses on the set operators if you have duplicates. The good news is that these operators work with rows and not at the column level, so this template will generalize to any pairs of union-compatible tables. You do not have to know the column names. About the author Joe Celko is a noted consultant, lecturer, teacher and one of the most-read SQL authors in the world.
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Joe Celko's SQL Puzzles and Answers
Can you solve Joe Celko's SQL puzzles?