How to cast a list of values within xquery in SQL Server?

Question

I have the following T-SQL code that I am trying to get to work:

IF EXISTS (SELECT * FROM sys.xml_schema_collections WHERE name = 'TST') DROP XML SCHEMA COLLECTION TST; 
GO
CREATE XML SCHEMA COLLECTION TST AS N'<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"><xsd:element name="root"><xsd:complexType><xsd:complexContent><xsd:restriction base="xsd:anyType"><xsd:sequence><xsd:element name="val" minOccurs="0" maxOccurs="unbounded" type="xsd:string" /></xsd:sequence></xsd:restriction></xsd:complexContent></xsd:complexType></xsd:element></xsd:schema>';
GO
DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT @xml.value('sum(//val)','INT')

The xsd tells us that the val node is of type string. Therefor the sum() function won't work:

Msg 9308, Level 16, State 1, Line 7
XQuery [value()]: The argument of 'sum()' must be of a single numeric primitive type or 'http://www.w3.org/2004/07/xpath-datatypes#untypedAtomic'. Found argument of type 'xs:string *'.

I have tried the following ways to get this to work:

---

Option 1:

One work-around is to just get rid of the xsd schema:

DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT (CAST(@xml AS XML)).value('sum(//val)','INT')

That returns the correct result: 6. However, it seems rather crude and might not always be an option (e.g. when dealing with XML indexes).

---

(Non-)Option 2:

An alternative is, to cast each value to xs:integer before summing them together. However, that is where I am stuck.

It is easy to cast a single element:

DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT @xml.value('sum(xs:integer((//val)[1]))','INT')

But that results in 1, which is the wrong result. This is expected, as now (because of the [1]) only the first val node gets looked at.

---

(Non-)Option 3:

There are two ways to write a cast in xquery:

DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT @xml.value('sum(xs:integer((//val)))','INT')
GO
DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT @xml.value('sum(((//val) cast as xs:integer ?))','INT')

However, both work only with singletons as input:

Msg 2365, Level 16, State 1, Line 15
XQuery [value()]: Cannot explicitly convert from 'xs:string *' to 'xs:integer'
Msg 2365, Level 16, State 1, Line 18
XQuery [value()]: Cannot explicitly convert from 'xs:string *' to 'xs:integer ?'

---

(Non-)Option 4:

According to the xquery documentation you should be able to do this:

DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT @xml.value('sum((//val)! xs:integer(.))','INT')

However, that is seems to not be implemented by SQL Server:

Msg 2217, Level 16, State 1, Line 21
XQuery [value()]: ',' or ')' expected

---

Option 5:

You could use the .nodes() function and solve the problem outside of xquery:

DECLARE @xml XML(TST) = '<root><val>1</val><val>2</val><val>3</val></root>';
SELECT SUM(X.V.value('.','INT')) FROM @xml.nodes('//val') X(V)

This results in the correct value of 6. However, I am looking for a solution within xquery, so this is not an option for me.

---

Option ?:

With that I am out of ideas. Is there a solution to this problem?

---

---

Extra:

In addition, I would really like to know what the ? stands for in this line, taken from one of the examples above:

SELECT @xml.value('sum(((//val)[1] cast as xs:integer ?))','INT')

SQL Server requires it. You get this error, if you try without:

Msg 9301, Level 16, State 1, Line 18
XQuery [value()]: In this version of the server, 'cast as <type>' is not available. Please use the 'cast as <type> ?' syntax.

This error hints at the fact, that this requirement is a limitation of SQL Server. However, I was not able to find documentation about its meaning anywhere.

• Mr.Brownstone: Is there a reason that you cannot change the schema to be an integer instead of a string? The question mark btw means that the sequence may be empty: w3.org/TR/xpath-functions-30 section 1.4
• Sebastian Meine: @Mr.Brownstone, thanks for the ?. That makes a lot of sense. See also: w3.org/TR/xpath-30/#prod-xpath30-OccurrenceIndicator
• Sebastian Meine: @Mr.Brownstone, in the current setting I do not have control over the schema. So, I cannot change it.

Answer 1

You can do it in xQuery with a FLWOR Statement and Iteration (XQuery) using cast on each value returned.

SELECT @xml.value('sum(for $val in //val return $val cast as xs:int?)','INT')

A query like that could make you think that SQL Server will do some time consuming looping operations but if you have a look at the query plan you will see that is not the case. The for loop is transformed to a plan that gets all the values from a single call to a Table-valued function, converts them to an integer and then using an aggregate operator to calculate the sum.

enter image description here