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Joined 2026-09-21 Message
Incoming links: Database aggregation
Past exam of the computer science course of the University of Oxford 2015 Databases 1 a iii Solution 2026-10-31
An arithmetic mean is database aggregation, which is outside classical relational algebra. Use SQL:For non-null prices of qualifying sales, the result isEach sale contributes once, even when a car was sold on several invoices. Do not use
SELECT AVG(s.price) AS average_sale_price
FROM Sales AS s
JOIN Car AS c ON c.serialnumber = s.serialnumber
WHERE c.make = 'Saab';DISTINCT inside AVG: two different sales at the same price must both count. The car's database primary key makes the relational join match each sale to exactly one car, so the relational join introduces no extra weighting.If there are no qualifying non-null prices,
AVG returns SQL NULL. It ignores null prices if the implementation permits them; it does not replace an absent arithmetic mean with zero.[ref]Aggregate Functions in the PostgreSQL documentation. Section 9.21,
avg and the paragraph on aggregates with no selected rows Relational algebra 2026-10-31
An algebra of operations on database relations. Classical relational algebra has relational selection, relational projection, relational join, Cartesian product, union and relational difference; database aggregation requires an extension.
SQL NULL 2026-10-31
SQL's marker for an unknown or absent value. Database aggregation generally ignores these markers, and
AVG returns this marker when there are no non-null inputs.[ref]Aggregate Functions in the PostgreSQL documentation. Section 9.21, general-purpose aggregate functions and empty-input behavior