Requires A Singleton?

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Have you ever got this error message when you tried to parse an XML string in SQL Server?

XQuery [value()]: ‘value()’ requires a singleton (or empty sequence), found operand of type ‘xdt:untypedAtomic *’

 
How do we fix this?

This error is saying that the value() function needs an xpath that only exits once (non-repeating node). You and I know that there will only be one instance of , but SQL Server does not know that. To solve this, we just need to tell SQL Server that it should use the first instance of this AnimalName node that it finds. We do this by added the [1] tag to the xpath. We want to add it after the closing parentheses. This will tell SQL Server to grab whatever is the first node with that name.

 
Extra Fun

If there are multiple nodes with the same name in your XML, you can adjust the number inside the square braces to match whatever node you want. For example:

[1] would return Baboon
[2] would return Cat
[3] would return Dog

How To Pull Single Value From XML

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SQL Server has a built in method for easily pulling one value from an XML. To do this you use the method called .value(). The .value method will run an XQuery against the XML specified in the query. This method is scalar, so it will only return 1 value. You cannot use this to return multiple values.

In the query above, you can see that you can just pass the xpath and the datatype that you want the output to be. This will pull that value out of the XML and put it in the specified format. You can see that after the xpath, there is a funny syntax [1]. This tells SQL Server to grab the first instance of the AnimalName node. You need to do this because the .value() method only returns one value and will not work with a repeating node.

Reference: https://msdn.microsoft.com/en-us/library/ms178030.aspx

Parse XML With SQL Server

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SQL Server has a lot of useful functionality built-in when it comes to XML.  Today I want to show you an easy way to parse XML data in to a usable format inside your query.

 
For this example, I will be parsing XML data into a temp table, then inserting that data in to a table.

 
The statement that parses the XML has 2 parts to it.

This part says to parse the @XMLToParse variable (previously filled with the XML file) using the method built in to SQL Server called .nodes.  It specifies the repeating node to be Animals/LandAnimals/Animal.  It assigns an aliases this XML parsed records as a table named xmlData.  The (A) is the column name of the rowset.  This will be referenced in the select part of the statement.

 

The select part of this statement references xmlData (which is the table aliases) and A (which is the column named for that table).  It calls the .value function to return the value from the table/column.  For the .value function, you pass in 2 elements.

The first element is the field.  In this case we are passing in just a period.  We do this because there is no node below the Animal node.  If there was, we would need to specify it here..

The second parameter is the datatype that you would like the value to be casted as.  Then I always aliases the field to something relevant.

 
Although parsing XML can be a little confusing in SQL Server, it is very powerful.  This is a great way to pass bulk data to a stored procedure from any type of client application.