Updating the xml data nodes by passing the SQL variable - sql-server

Question

I have the following XML and I need to update the specified one based on the parameter:
<DOPremium>
<BasePremium>337500</BasePremium>
<TotalPremium>337500</TotalPremium>
<NettPremium>337500</NettPremium>
<GrossPremium>337500</GrossPremium>
<OptionId>0</OptionId>
</DOPremium>
<DOPremium>
<BasePremium>337500</BasePremium>
<TotalPremium>337500</TotalPremium>
<NettPremium>337500</NettPremium>
<GrossPremium>337500</GrossPremium>
<OptionId>1</OptionId>
</DOPremium>
<DOPremium>
<BasePremium>337500</BasePremium>
<TotalPremium>337500</TotalPremium>
<NettPremium>337500</NettPremium>
<GrossPremium>337500</GrossPremium>
<OptionId>2</OptionId>
</DOPremium>
I'm trying to update the respective nodes based on the selection of the DOPremium object, but I'm not able to do that. Can someone verify where I'm wrong?
SET #NewXmlValue = N' <BasePremium>[sql:variable("#R15_premium")]</BasePremium>'
SET #DataXml.modify('delete /*/Premiums/DOPremium/BasePremium[sql:variable("#OptionID")]')
SET #DataXml.modify('insert sql:variable("#NewXmlValue") into (/*/Premiums/DOPremium[sql:variable("#OptionID")])[1]')
-- Add TotalPremium
SET #NewXmlValue = N' <TotalPremium>[sql:variable("#R15_premium")]</TotalPremium>'
SET #DataXml.modify('delete /*/Premiums/DOPremium/TotalPremium[sql:variable("#OptionID")]')
SET #DataXml.modify('insert sql:variable("#NewXmlValue") into (/*/Premiums/DOPremium[sql:variable("#OptionID")])[1]')

Answer 1

OK, first of all, this can't work:
SET #NewXmlValue = N'<BasePremium>[sql:variable("#R15_premium")]</BasePremium>'
sql:variable() is only interpreted as a function in an XQuery operation. This isn't an XQuery operation, so it will just textually insert sql:variable(...). If you want an actual XML node with the text value of the variable, you have to be a little more roundabout:
SET #NewXmlValue = '';
SET #NewXmlValue = (SELECT #NewXmlValue.query('<BasePremium>{sql:variable("#R15_premium")}</BasePremium>'));
This approach (and others) can be found in the docs. (In this very simple case concatenating the strings in T-SQL also works, of course, but in general that's not a good idea because it doesn't take care of escaping the XML when necessary.)
The syntax for selecting the desired DOPremium node also needs work -- /BasePremium[sql:variable("#OptionID")] is legal, but it means "the BasePremium node that, sequentially numbering from 1, has number #OptionID". If #OptionID is supposed to match what's in OptionID, that's not the way to write it.
If your intent was to write "change the contents of the BasePremium value of the node with OptionID text equal to #OptionID to the value #R15_premium", here's how you do that (well, one way to do that):
SET #DataXml.modify('
replace value of (
/*
/Premiums
/DOPremium[child::OptionId/.=sql:variable("#OptionID")]
/BasePremium
/text()
)[1]
with sql:variable("#R15_premium")')
And something similar for TotalPremium. You can, of course, also replace entire nodes, but that seems unnecessary here.