Here are some tweaked examples & better problem definition:
Given a database created from catalog.xml below, what's the best way to update & add records received daily as in file daily-update.xml?
For every <ENTRY> node in daily-update.xml
-- If ENTRY/SKU found in catalog.xml replace the entire <ENTRY> node in catalog.xml with the one from daily-update.xml
-- If ENTRY/SKU not found in catalog.xml add the <ENTRY> node from daily-update.xml to catalog.xml
catalog.xml
<?xml version="1.0" encoding="UTF-8"?>
<CATALOG>
<ENTRIES>
<ENTRY>
<SKU>1</SKU>
<TITLE>Empire Burlesque</TITLE>
<ARTIST>Dylan</ARTIST>
<PRICE>10.90</PRICE>
</ENTRY>
<ENTRY>
<SKU>2</SKU>
<TITLE>Hide your heart</TITLE>
<ARTIST>Bonnie Tyler</ARTIST>
<PRICE>9.90</PRICE>
</ENTRY>
</ENTRIES>
</CATALOG>
daily-update.xml
<?xml version="1.0" encoding="UTF-8"?>
<CATALOG>
<ENTRIES>
<ENTRY>
<SKU>1</SKU>
<TITLE>Empire Burlesque</TITLE>
<ARTIST>Bob Dylan</ARTIST>
<PRICE>29.99</PRICE>
<NEWTHING>some value</NEWTHING>
</ENTRY>
<ENTRY>
<SKU>5</SKU>
<TITLE>Tupelo Honey</TITLE>
<ARTIST>Van Morrison</ARTIST>
<PRICE>8.20</PRICE>
</ENTRY>
</ENTRIES>
</CATALOG>
catalog.xml after daily update:
<CATALOG>
<ENTRIES>
<ENTRY>
<SKU>1</SKU>
<TITLE>Empire Burlesque</TITLE>
<ARTIST>Bob Dylan</ARTIST>
<PRICE>29.99</PRICE>
<NEWTHING>some value</NEWTHING>
</ENTRY>
<ENTRY>
<SKU>2</SKU>
<TITLE>Hide your heart</TITLE>
<ARTIST>Bonnie Tyler</ARTIST>
<PRICE>9.90</PRICE>
</ENTRY>
<ENTRY>
<SKU>5</SKU>
<TITLE>Tupelo Honey</TITLE>
<ARTIST>Van Morrison</ARTIST>
<PRICE>8.20</PRICE>
</ENTRY>
</ENTRIES>
</CATALOG>