Convert XML to CSV (Pipe Delimited) - Large Files 100mb to 1GB

I am working with large xml files (ranging from 100mb to 1GB+), thus it is not really possible to load them into memory.. and the goal is to convert these files to a csv format, but pipe (|) delimited.


The XML file looks like this;


  <?xml version="1.0" encoding="UTF-8" ?>
- <merch_item_feed>
- <item_data>
- <item_basic_data>
  <item_unique_id>0115526102</item_unique_id>
  <item_ean>9780115526107</item_ean>
  <item_sku>0115526102</item_sku>
  <item_upc />
  <item_mpn />
  <item_brand>Stationery Office Books</item_brand>
  <item_name>The Official Learning to Drive Pack (Driving Skills)</item_name>
  <item_model />
  <item_category>Book</item_category>
  <item_short_desc>Paperback, Stationery Office Books</item_short_desc>
  <item_page_url>http://www.amazon.co.uk/exec/obidos/ASIN/0115526102/AssocID/ref=nosim</item_page_url>
  <amzn_page_url>http://www.amazon.co.uk/exec/obidos/ASIN/0115526102/AssocID/ref=nosim</amzn_page_url>
  <offer_page_url>http://www.amazon.co.uk/o/redirect?tag=AssocID&link_code=asm&path=tg/detail/offer-listing/-/0115526102/new/ASIN/0115526102&camp=1634&creative=6738</offer_page_url>
  <offer_used_url>http://www.amazon.co.uk/o/redirect?tag=AssocID&link_code=asm&path=tg/detail/offer-listing/-/0115526102/used/ASIN/0115526102&camp=1634&creative=6738</offer_used_url>
  <item_image_url>http://ec1.images-amazon.com/images/I/31DY01WGJ1L.jpg</item_image_url>
  <item_image_url_small>http://ec1.images-amazon.com/images/I/11FE11DK7YL.jpg</item_image_url_small>
  <item_salesrank>233499</item_salesrank>
  <item_price>21.23</item_price>
  <item_inventory>Usually dispatched within 1-2 business days</item_inventory>
  <item_shipping_charge>Check Site.</item_shipping_charge>
  <amzn_price>24.99</amzn_price>
  <amzn_inventory>Usually dispatched within 24 hours</amzn_inventory>
  <amzn_shipping_charge>Free!</amzn_shipping_charge>
  <fm_price>24.99</fm_price>
  <fm_inventory>Usually dispatched within 24 hours</fm_inventory>
  <fm_shipping_charge>Free!</fm_shipping_charge>
  <tp_new_price>21.23</tp_new_price>
  <tp_new_inventory>Usually dispatched within 1-2 business days</tp_new_inventory>
  <tp_new_shipping_charge>Check Site.</tp_new_shipping_charge>
  <tp_used_price>20.00</tp_used_price>
  <tp_used_inventory>In Stock</tp_used_inventory>
  <tp_used_shipping_charge>Check Site.</tp_used_shipping_charge>
  </item_basic_data>
- <prod_specific_data category="book">
  <known_attr_val_pair attr="book_author" val="Driving Standards Agency" />
  <known_attr_val_pair attr="book_isbn" val="0115526102" />
  <known_attr_val_pair attr="book_format" val="Paperback" />
  </prod_specific_data>
- <merch_cat_list>
- <merch_cat_item>
  <merch_cat_name>277082</merch_cat_name>
  <merch_cat_path>Books/Subjects/Reference/Transport/Automotive/Driving & the Highway Code</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>278131</merch_cat_name>
  <merch_cat_path>Books/Subjects/Science & Nature/Engineering & Technology/Civil Engineering/Road & Transport</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>10834521</merch_cat_name>
  <merch_cat_path>Books/Special Features/34% off Books over £10/Science & Nature</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>10834491</merch_cat_name>
  <merch_cat_path>Books/Special Features/34% off Books over £10/Reference & Languages</merch_cat_path>
  </merch_cat_item>
  </merch_cat_list>
  </item_data>
- <item_data>
- <item_basic_data>
  <item_unique_id>0115528423</item_unique_id>
  <item_ean>9780115528422</item_ean>
  <item_sku>0115528423</item_sku>
  <item_upc />
  <item_mpn />
  <item_brand>The Stationary Office (TSO)</item_brand>
  <item_name>The Official DSA Theory Test for Motorcyclists CD-ROM</item_name>
  <item_model />
  <item_category>Software</item_category>
  <item_short_desc>, Platforms: Windows XP</item_short_desc>
  <item_page_url>http://www.amazon.co.uk/exec/obidos/ASIN/0115528423/AssocID/ref=nosim</item_page_url>
  <amzn_page_url>http://www.amazon.co.uk/exec/obidos/ASIN/0115528423/AssocID/ref=nosim</amzn_page_url>
  <offer_page_url>http://www.amazon.co.uk/o/redirect?tag=AssocID&link_code=asm&path=tg/detail/offer-listing/-/0115528423/new/ASIN/0115528423&camp=1634&creative=6738</offer_page_url>
  <offer_used_url>http://www.amazon.co.uk/o/redirect?tag=AssocID&link_code=asm&path=tg/detail/offer-listing/-/0115528423/used/ASIN/0115528423&camp=1634&creative=6738</offer_used_url>
  <item_image_url>http://ec1.images-amazon.com/images/I/31Au9yM7IZL.jpg</item_image_url>
  <item_image_url_small>http://ec1.images-amazon.com/images/I/11ldivFAIML.jpg</item_image_url_small>
  <item_salesrank>1068</item_salesrank>
  <item_price>16.99</item_price>
  <item_inventory>Not yet released</item_inventory>
  <item_shipping_charge>Free!</item_shipping_charge>
  <amzn_price>16.99</amzn_price>
  <amzn_inventory>Not yet released</amzn_inventory>
  <amzn_shipping_charge>Free!</amzn_shipping_charge>
  <fm_price>16.99</fm_price>
  <fm_inventory>Not yet released</fm_inventory>
  <fm_shipping_charge>Free!</fm_shipping_charge>
  </item_basic_data>
- <prod_specific_data category="software">
  <known_attr_val_pair attr="hardware_platform" val="PC" />
  <known_attr_val_pair attr="software_os" val="Windows XP" />
  <known_attr_val_pair attr="software_format" val="CD-ROM" />
  </prod_specific_data>
- <merch_cat_list>
- <merch_cat_item>
  <merch_cat_name>277082</merch_cat_name>
  <merch_cat_path>Books/Subjects/Reference/Transport/Automotive/Driving & the Highway Code</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>278131</merch_cat_name>
  <merch_cat_path>Books/Subjects/Science & Nature/Engineering & Technology/Civil Engineering/Road & Transport</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>912026</merch_cat_name>
  <merch_cat_path>Software/Categories/Hobbies & Pastimes/Driving Tests</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>16305411</merch_cat_name>
  <merch_cat_path>Software/Categories/Hobbies & Pastimes/All Hobbies & Pastimes</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>317243011</merch_cat_name>
  <merch_cat_path>Software/Categories/Digital Imaging/Creativity Software</merch_cat_path>
  </merch_cat_item>
- <merch_cat_item>
  <merch_cat_name>341610011</merch_cat_name>
  <merch_cat_path>uk-shops/Education Resources/Software/Driving Tests</merch_cat_path>
  </merch_cat_item>
  </merch_cat_list>
  </item_data>
- </merchitemfeed>


Objective is to extract the data from the 'item_basic_data' elements and separate them by pipe character.

Output should look something like (with the field headers);

item_unique_id|item_ean|iteam_upc
12345678901|12345678|12345678
12345678901|12345678|12345678
12345678901|12345678|12345678

------------------------------------------

Please note that only the information from 'item_basic_data' needs to be extracted - instructions on how to accomplish this is sufficient as an answer.  However, if you know your stuff, I would appreciate a solution that could extract the first instance of 'merch_cat_path'.  If you notice, each 'item_basic_data' has 4 or 5 duplicate elements of 'merch_cat_path', but we only want the first instance if possible.

I am assuming we will need some xslt file, but I don't know how to write it.  I am experimenting with a program that will do the processing of the input xml, transform xslt, and output csv files, but it does not supply the xslt file itself.

Also, if you have any suggestions for similar programs that can handle & process large xml files - preferably freeware, but commercial is ok too.