Sorting Hostnames in Domain Order

6.15.1 Problem

You want to sort hostnames in domain order, with the rightmost parts of the names more significant than the leftmost parts.

6.15.2 Solution

Break apart the names and sort the pieces from right to left.

6.15.3 Discussion

Hostnames are strings and therefore their natural sort order is lexical. However, it's often desirable to sort hostnames in domain order, where the rightmost segments of the hostname values are more significant than the leftmost segments. Suppose you have a table hostname that contains the following names:

mysql> SELECT name FROM hostname ORDER BY name;
+--------------------+
| name |
+--------------------+
| cvs.php.net |
| dbi.perl.org |
| jakarta.apache.org |
| lists.mysql.com |
| mysql.com |
| www.kitebird.com |
+--------------------+

The preceding query demonstrates the natural lexical sort order of the name values. That differs from domain order, as shown by the following table:

Lexical order

Domain order

cvs.php.net

www.kitebird.com

dbi.perl.org

mysql.com

jakarta.apache.org

lists.mysql.com

lists.mysql.com

cvs.php.net

mysql.com

jakarta.apache.org

www.kitebird.com

dbi.perl.org

Producing domain-ordered output is a substring-sorting problem, where it's necessary to extract each segment of the names so they can be sorted in right-to-left fashion. There is also an additional complication if your values contain different numbers of segments, as our example hostnames do. (Most of them have three segments, but mysql.com has only two.)

To extract the pieces of the hostnames, begin by using SUBSTRING_INDEX( ) in a manner similar to that described previously in Recipe 6.14. The hostname values have a maximum of three segments, from which the pieces can be extracted left to right like this:

SUBSTRING_INDEX(SUBSTRING_INDEX(name,'.',-3),'.',1)
SUBSTRING_INDEX(SUBSTRING_INDEX(name,'.',-2),'.',1)
SUBSTRING_INDEX(name,'.',-1)

These expressions work properly as long as all the hostnames have three components. But if a name has fewer than three, we don't get the correct result, as the following query demonstrates:

mysql> SELECT name,
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(name,'.',-3),'.',1) AS leftmost,
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(name,'.',-2),'.',1) AS middle,
 -> SUBSTRING_INDEX(name,'.',-1) AS rightmost
 -> FROM hostname;
+--------------------+----------+----------+-----------+
| name | leftmost | middle | rightmost |
+--------------------+----------+----------+-----------+
| cvs.php.net | cvs | php | net |
| dbi.perl.org | dbi | perl | org |
| lists.mysql.com | lists | mysql | com |
| mysql.com | mysql | mysql | com |
| jakarta.apache.org | jakarta | apache | org |
| www.kitebird.com | www | kitebird | com |
+--------------------+----------+----------+-----------+

Notice the output for the mysql.com row; it has mysql for the value of the leftmost column, where it should have an empty string. The segment-extraction expressions work by pulling off the rightmost n segments, then returning the leftmost segment of the result. The source of the problem for mysql.com is that if there aren't n segments, the expression simply returns the leftmost segment of however many there are. To fix this problem, prepend a sufficient number of periods to the hostname values to guarantee that they have the requisite number of segments:

mysql> SELECT name,
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT('..',name),'.',-3),'.',1)
 -> AS leftmost,
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT('.',name),'.',-2),'.',1)
 -> AS middle,
 -> SUBSTRING_INDEX(name,'.',-1) AS rightmost
 -> FROM hostname;
+--------------------+----------+----------+-----------+
| name | leftmost | middle | rightmost |
+--------------------+----------+----------+-----------+
| cvs.php.net | cvs | php | net |
| dbi.perl.org | dbi | perl | org |
| lists.mysql.com | lists | mysql | com |
| mysql.com | | mysql | com |
| jakarta.apache.org | jakarta | apache | org |
| www.kitebird.com | www | kitebird | com |
+--------------------+----------+----------+-----------+

That's pretty ugly. But these expressions do serve to extract the substrings that are needed for sorting hostname values correctly in right-to-left fashion:

mysql> SELECT name FROM hostname
 -> ORDER BY
 -> SUBSTRING_INDEX(name,'.',-1),
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT('.',name),'.',-2),'.',1),
 -> SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT('..',name),'.',-3),'.',1);
+--------------------+
| name |
+--------------------+
| www.kitebird.com |
| mysql.com |
| lists.mysql.com |
| cvs.php.net |
| jakarta.apache.org |
| dbi.perl.org |
+--------------------+

If you had hostnames with a maximum of four segments rather than three, you'd need to add to the ORDER BY clause another SUBSTRING_INDEX( ) expression that prepends three dots to the hostname values.

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References



MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

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