Combination of detailed and aggregated data |
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Let the maximal and minimal prices among all the printers be output along with model and price of each printer.
But if we wouldn't use grouping, only the maximal and minimal values could be obtained, because standard syntax forbids (in view of ambiguous treatment of a result) usage of summarized values along with detailed ones without grouping by them: The solution to the problem is fairly simple and not a single. For example, subqueries in SELECT clause can be used. Such a subquery should return a single value, but not a set, as is the case: Solution 1
More effective approach is to use subquery for calculation of aggregates in FROM clause along with cartesian product. You should not afraid cartesian product in this case because the subquery returns only one row which will be joined each row of detailed data. Solution 2
Why do we state that the second solution has better performance in respect to the first one? The matter is that the subquery will be calculated twice in the first query whereas once in the second one. Besides, if the optimizer is not "clever" enough, the subqueries in the first solution will be calculated for each row of detailed data. Inspect the execution plans for your DBMS. Let's consider the task where aggregates depend on current row of detailed data, for example: Determine model and price of each printer as well as the maximal and minimal prices for printers of the same type. We shall try to adapt the above approaches for solving this problem. Subqueries in Solution 1 should be rewritten as correlated ones: Solution 1M
As for Solution 2, we might take non-standard join CROSS APPLY (SQL Server), which uses correlated subquery in FROM clause. Solution 2M
The type column has been added in the solutions 1M and 2M for better insight into. |
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