|You have an election database with tables listing political parties, election districts, and candidates running for parties in those districts. You want to know which parties have candidates running in all districts. Under Aggregates we show a |
If there are reasons not to aggregate, relational division can solve the problem. The basic idea in relational division is that, aside from aggregation, SQL has no direct way to express "all Xs for which all Y are Z", but does have a
SELECT DISTINCT party FROM parties WHERE NOT EXISTS ( SELECT * FROM districts WHERE NOT EXISTS ( SELECT * FROM candidates WHERE candidates.party=parties.party AND candidates.district=districts.district ) );Why is it called relational division? See the All possible recipes with given ingredients entry. Here the dividend is candidates, the divisor is districts and the quotient is a party count.
SELECT p.party FROM parties p LEFT JOIN ( SELECT a.party FROM ( SELECT DISTINCT party,district FROM parties CROSS JOIN districts ) a LEFT JOIN candidates c ON a.party=c.party AND a.district=c.district WHERE c.party IS NULL ) b ON p.party=b.party WHERE b.party IS NULL;Like numeric division, relational division has a gotcha: divide by zero. If the divisor table has zero rows, the quotient counts all distinct dividend instances. If that is not what you want, use aggregation.
Most "all Xs for which all Y are Z" queries can be written in any of these three ways. Try each one to see which performs best for your problem.