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Exercise #10

Find the printers having the highest price. Result set: model, price.

The problem usually does not cause difficulties, however, sometimes I receive by email solutions similar to the following one:

Console
Execute
  1. SELECT model, MAX(DISTINCT price)
  2. FROM Printer
  3. GROUP BY model

Clearly natural desire to solve a problem without subqueries. If it was required to deduce only a highest price, the grouping would be unnecessary as the maximum would be on all set of printers:

Console
Execute
  1. SELECT MAX(price)
  2. FROM Printer

However this exercise requires to deduce also number (numbers) of the model having a highest price. As we cannot use in SELECT clause the aggregate values alongside with detailed ones (without grouping by the detailed values), as a result we get the above wrong solution that use grouping by model. This solution gives a highest price over each model number, but we need to receive models, which have absolute (over all set of printers) highest price.

So, we need to use a subquery which calculates highest price:

Console
Execute
  1. SELECT model, price  
  2. FROM Printer  
  3. WHERE price = (SELECT MAX(price) 
  4.                FROM Printer
  5.                )

In so doing, the subquery can be entered not only with the simple comparison operator («=»), but also with IN or >=ALL clauses.

We can use subquery in the FROM clause also:

Console
Execute
  1. SELECT model, price 
  2. FROM Printer pr, (SELECT MAX(price) AS maxprice  
  3.                   FROM Printer
  4.                   ) AS mp 
  5. WHERE price = mp.maxprice

However it does not give a gain in performance because the calculation of a subquery is carried out only once in any case, thereafter the comparison of the prices for every row will be made.

But still, whether it is possible to solve a problem without a subquery?

T&S

To solve the problem on SQL-EX.RU

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