Functions on Sequences - distinct-values
Applies to: 
SQL Server
Removes duplicate values from the sequence specified by $arg. If $arg is an empty sequence, the function returns the empty sequence.
Syntax
fn:distinct-values($arg as xdt:anyAtomicType*) as xdt:anyAtomicType*
Arguments
$arg
Sequence of atomic values.
Remarks
All types of the atomized values that are passed to distinct-values() have to be subtypes of the same base type. Base types that are accepted are the types that support the eq operation. These types include the three built-in numeric base types, the date/time base types, xs:string, xs:boolean, and xdt:untypedAtomic. Values of type xdt:untypedAtomic are cast to xs:string. If there is a mixture of these types, or if other values of other types are passed, a static error is raised.
The result of distinct-values() receives the base type of the passed in types, such as xs:string in the case of xdt:untypedAtomic, with the original cardinality. If the input is statically empty, empty is implied and a static error is raised.
Values of type xs:string are compared to the XQuery default Unicode Codepoint Collation.
Examples
This topic provides XQuery examples against XML instances that are stored in various xml type columns in the AdventureWorks database.
A. Using the distinct-values() function to remove duplicate values from the sequence
In this example, an XML instance that contains telephone numbers is assigned to an xml type variable. The XQuery specified against this variable uses the distinct-values() function to compile a list of telephone numbers that do not contain duplicates.
declare @x xml
set @x = '<PhoneNumbers>
<Number>111-111-1111</Number>
<Number>111-111-1111</Number>
<Number>222-222-2222</Number>
</PhoneNumbers>'
-- 1st select
select @x.query('
distinct-values( data(/PhoneNumbers/Number) )
') as result
This is the result:
111-111-1111 222-222-2222
In the following query, a sequence of numbers (1, 1, 2) is passed in to the distinct-values() function. The function then removes the duplicate in the sequence and returns the other two.
declare @x xml
set @x = ''
select @x.query('
distinct-values((1, 1, 2))
') as result
The query returns 1 2.
Implementation Limitations
These are the limitations:
The distinct-values() function maps integer values to xs:decimal.
The distinct-values() function only supports the previously mentioned types and does not support the mixture of base types.
The distinct-values() function on xs:duration values is not supported.
The syntactic option that provides a collation is not supported.