How to update specific XML attributes in SQL Server?

Question

We are storing an XML document of all the different costs in SQL Server 2008 r2. Now, certain prices need to be update based on a formula.

One approach to this would be to:

1. Shred the XML and access the values.
2. Do the calculation
3. Now rebuild the XML structure.

Is this the only way to do it? Or is there an easier way where we can access the values on the fly and update the entire XML in one go? For this kind of scenario, what is the best approach?

Here is the sample of the XML:

<ItemAttributeData>
 <Section ID="469919" TypeID="61000002">
  <Attributes>
    <Attribute ID="3870" UniqueID="1c71212b-8395-4b43-a44d-9a63374c8c99" VALUE="2" />
    <Attribute ID="3569" UniqueID="c0b754d3-e03e-4d4a-9bc1-dcb11c04535b" VALUE="2" />
    <Attribute ID="1592" UniqueID="2609b344-88b2-44d6-be20-07b634705c30" VALUE="2" />
   </Attributes>
  </Section>
 <Section ID="469920" TypeID="61000001">
   <Attributes>
     <Attribute ID="3702" UniqueID="7cabf9e8-4f60-4e1e-afe5-75b153e8c5eb" VALUE="1000001649" />
     <Attribute ID="3870" UniqueID="595f8e2c-bccb-4fcb-81d4-303fcb7d44c4" VALUE="2" />
     <Attribute ID="3868" UniqueID="2226f6c6-380a-4195-98f7-c6deec84e4e8" VALUE="2" />
     <Attribute ID="3754" UniqueID="bf936c3f-fa96-49b9-8a62-46fb7e0009d5" VALUE="1000001600" />
     <Attribute ID="2204" UniqueID="bdc78784-86ad-4567-bc06-40896d603eaf" VALUE="1" />
     <Attribute ID="3874" UniqueID="8728249f-5e60-4938-a60d-208426fe11c8" VALUE="1" />
    </Attributes>
   </Section>
  <Section ID="469921" TypeID="61000001" />
 </ItemAttributeData>

Answer 1

You can use non standard XML update command replace value of . Like

DECLARE @myDoc xml
SET @myDoc = '<Root>
<Location LocationID="10" 
            LaborHours=".1"
            MachineHours=".2" >Manu steps are described here.
<step>Manufacturing step 1 at this work center</step>
<step>Manufacturing step 2 at this work center</step>
</Location>
</Root>'
--SELECT @myDoc

SET @myDoc.modify('
  replace value of (/Root/Location[1]/@LaborHours)[1]
  with (
       if (count(/Root/Location[1]/step) > 3) then
         "3.0"
       else
          "1.0"
      )
')
SELECT @myDoc