Section 3.6. Ranking Regions by Size

3.6 Ranking Regions by Size

3.6.1 Problem

You want to list all regions in the table, and you want to list them according to their size. With respect to our example, you wish to list all regions of two or more containers with a purity of 100, and you wish to sort that list by the number of containers in each region.

3.6.2 Solution

Use the following query to produce the desired list:

 SELECT     p1.ContainerId RegBeg, p2.ContainerId RegEnd,     p2.ContainerId-p1.ContainerId+1 RegionSize FROM ProductionLine p1, ProductionLine p2 WHERE (p1.ContainerId < p2.ContainerId) AND    NOT EXISTS(SELECT * FROM ProductionLine p3        WHERE (p3.Purity!=100 AND        p3.ContainerId BETWEEN p1.ContainerId AND p2.ContainerId)        OR (p3.ContainerId=p1.ContainerId-1 AND p3.Purity=100)        OR (p3.ContainerId=p2.ContainerId+1 AND p3.Purity=100)) ORDER BY p2.ContainerId-p1.ContainerId DESC RegBeg      RegEnd      RegionSize   ----------- ----------- -----------  10          12          3 1           2           2 

3.6.3 Discussion

As you can see, this query is similar to the one that is used to find regions. The added feature is the ORDER BY clause, which sorts the regions according to their size. It relies on the fact that the table uses an arithmetically increasing index through which the size of a region can be calculated based on the difference between the two indices making up the region's borders.

Rather than just report the beginning and ending index for each region, this query uses the same calculation in the SELECT list as in the ORDER BY clause to report the size of each region in terms of the number of containers.

The query comes in handy when you have to prepare data for a best-fitting algorithm, and you wish to use the database to presort the data.

You can expand on the solution shown in this recipe, if you like, to show the smallest available region that is still larger than a given size. To do this, add a WHERE clause expression to limit the size of the regions that are sorted. For example:

 SELECT TOP 1    p1.ContainerId RegBeg, p2.ContainerId RegEnd,     p2.ContainerId-p1.ContainerId+1 RegionSize FROM ProductionLine p1, ProductionLine p2 WHERE     (p1.ContainerId < p2.ContainerId) AND  (p2.ContainerId-p1.ContainerId)>=2 AND  NOT EXISTS(SELECT * FROM ProductionLine p3        WHERE (p3.Purity!=100 AND        p3.ContainerId BETWEEN p1.ContainerId AND p2.ContainerId)        OR (p3.ContainerId=p1.ContainerId-1 AND p3.Purity=100)        OR (p3.ContainerId=p2.ContainerId+1 AND p3.Purity=100)) ORDER BY p2.ContainerId-p1.ContainerId ASC 

This query returns the smallest possible region that still fits into the limit. In this case, only the first region that fits the limitations is returned.