Recipe 10.39. Exporting Query Results as XML

Problem

You want to export the result of a query as an XML document.

Solution

mysql can do that, or you can write your own exporter.

Discussion

You can use mysql to produce XML-format output from a query result (Section 1.20).

You can also write your own XML-export program. One way to do this is to issue the query and then write it out, adding all the XML markup yourself. But it's easier to install a few Perl modules and let them do the work:

• XML::Generator::DBI issues a query over a DBI connection and passes the result to a suitable output writer.

• XML::Handler::YAWriter provides one such writer.

The following script, mysql_to_xml.pl, is somewhat similar to mysql_to_text.pl (Section 10.17), but doesn't take options for such things as the quote or delimiter characters. They are unneeded for reading XML, because that is done by standard XML parsing routines. The options that mysql_to_xml.pl does understand are:

--execute = query, -e query

Execute query, and export its output.

--table = tbl_name, -t tbl_name

Export the contents of the named table. This is equivalent to using --execute to specify a query value of SELECT * FROM tbl_name.

If necessary, you can also specify standard connection parameter options such as --user or --host. The final argument on the command line should be the database name, unless it's implicit in the query.

Suppose that you want to export the contents of an experimental-data table expt that looks like this:

mysql> SELECT * FROM expt; +---------+------+-------+ | subject | test | score | +---------+------+-------+ | Jane    | A    |    47 | | Jane    | B    |    50 | | Jane    | C    |  NULL | | Jane    | D    |  NULL | | Marvin  | A    |    52 | | Marvin  | B    |    45 | | Marvin  | C    |    53 | | Marvin  | D    |  NULL | +---------+------+-------+ 

To do that, invoke mysql_to_xml.pl using either of the following commands:

% mysql_to_xml.pl --execute="SELECT * FROM expt" cookbook > expt.xml % mysql_to_xml.pl --table=cookbook.expt > expt.xml             

The resulting XML document, expt.xml, looks like this:

<?xml version="1.0" encoding="UTF-8"?> <rowset>  <select query="SELECT * FROM expt">   <row>    <subject>Jane</subject>    <test>A</test>    <score>47</score>   </row>   <row>    <subject>Jane</subject>    <test>B</test>    <score>50</score>   </row>   <row>    <subject>Jane</subject>    <test>C</test>   </row>   <row>    <subject>Jane</subject>    <test>D</test>   </row>   <row>    <subject>Marvin</subject>    <test>A</test>    <score>52</score>   </row>   <row>    <subject>Marvin</subject>    <test>B</test>    <score>45</score>   </row>   <row>    <subject>Marvin</subject>    <test>C</test>    <score>53</score>   </row>   <row>    <subject>Marvin</subject>    <test>D</test>   </row>  </select> </rowset> 

Each table row is written as a <row> element. Within a row, column names and values are used as element names and values, one element per column. Note that NULL values are omitted from the output.

The script produces this output with very little code after it processes the command-line arguments and connects to the MySQL server. The XML-related parts of mysql_to_xml.pl are the use statements that pull in the necessary modules and the code to set up and use the XML objects. Given a database handle $dbh and a query string $query, there's not a lot to this process. The code instructs the writer object to send its results to the standard output, and then connects that object to DBI and issues the query:

#!/usr/bin/perl # mysql_to_xml.pl - given a database and table name, # dump the table to the standard output in XML format. use strict; use warnings; use DBI; use XML::Generator::DBI; use XML::Handler::YAWriter; # ... process command-line options (not shown) ... # ... connect to database (not shown) ... # create output writer; "-" means "standard output" my $out = XML::Handler::YAWriter->new (AsFile => "-"); # set up connection between DBI and output writer my $gen = XML::Generator::DBI->new (               dbh => $dbh,            # database handle               Handler => $out,        # output writer               RootElement => "rowset" # document root element             ); # issue query and write XML $gen->execute ($stmt); $dbh->disconnect (); 

Other languages might have library modules to perform similar XML export operations. For example, the Ruby DBI::Utils::XMLFormatter module has a table method that easily exports a query result as XML. Here's a simple script that uses it:

#!/usr/bin/ruby -w # xmlformatter.rb - demonstrate DBI::Utils::XMLFormatter.table method require "Cookbook" stmt = "SELECT * FROM expt" # override statement with command line argument if one was given stmt = ARGV[0] if ARGV.length > 0 begin   dbh = Cookbook.connect rescue DBI::DatabaseError => e   puts "Could not connect to server"   puts "Error code: #{e.err}"   puts "Error message: #{e.errstr}" end DBI::Utils::XMLFormatter.table(dbh.select_all(stmt)) dbh.disconnect