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SQL Problems and solutions S. I. Moiseenko

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Aggregate function to aggregate function page 1

Let us consider this exercise:

Find the maximum value among the average prices of PCs, counted separately for each manufacturer.

Calculation of average cost for makers is not difficult:

Console

Execute

SELECT AVG(price) avg_price
FROM Product P JOIN PC ON P.model = PC.model 
GROUP BY maker;

However, the standard prohibits the use of a subquery as an argument to aggregate function, i.e. we can not solve the exercise by this way:

Console

Execute

SELECT MAX(
SELECT AVG(price) avg_price
FROM Product P JOIN PC ON P.model = PC.model 
GROUP BY maker
);

In such cases, use a subquery in the FROM clause:

Console

Execute

SELECT MAX(avg_price) 
FROM (SELECT AVG(price) avg_price
      FROM Product P JOIN PC ON P.model = PC.model 
      GROUP BY maker
     ) X;

By means of new features of language - window functions - this problem can be solved without a subquery:

Console

Execute

SELECT DISTINCT MAX(AVG(price)) OVER () max_avg_price
      FROM Product P JOIN PC ON P.model = PC.model 
     GROUP BY maker;

Note that window functions admit use of aggregate function as argument. DISTINCT keyword is necessary here because the maximal value, which has been counted up over all set of average values, will be "attributed" to each manufacturer.

We can't use aggregate function as a parameter of another aggregate function. This implies impossibility of the solution to the above problem as follows.

SELECT MAX(AVG(price)) max_avg_price
    FROM Product P JOIN PC ON P.model = PC.model 
    GROUP BY maker;

But there are no rules without exceptions. As it seems to be unreal, but this query is working in Oracle and returns the required result:

MAX_AVG_PRICE
850

You can assure yourself that it is true by visiting the learn-stage exercises page at sql-ex.ru, selecting Oracle in the DBMS list, and running the query (without checking solution for example).

Besides, the solution which uses window function will work in Oracle also. I can suggest that the solution without window function actually uses it implying the OVER() clause implicitly.

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Oracle AVG MAX aggregate functions window functions

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SQL Problems and solutions

S. I. Moiseenko

SELECT statement

Getting summarizing values

Aggregate function to aggregate function page 1