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Invoking CLR User-Defined Aggregate Functions

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In Transact-SQL SELECT statements, you can invoke common language runtime (CLR) user-defined aggregates, subject to all the rules that apply to system aggregate functions.

The following additional rules apply:

  • The current user must have EXECUTE permission on the user-defined aggregate.
  • User-defined aggregates must be invoked using a two-part name in the form of schema_name.udagg_name.
  • The argument type of the user-defined aggregate must match or be implicitly convertible to the input_type of the aggregate, as defined in the CREATE AGGREGATE statement.
  • The return type of the user-defined aggregate must match the return_type in the CREATE AGGREGATE statement.

Here is an example of a user-defined aggregate function that concatenates a set of string values taken from a column in a table:

[C#]

using System;
using System.Data;
using Microsoft.SqlServer.Server;
using System.Data.SqlTypes;
using System.IO;
using System.Text;

[Serializable]
[SqlUserDefinedAggregate(
    Format.UserDefined, //use clr serialization to serialize the intermediate result
    IsInvariantToNulls = true, //optimizer property
    IsInvariantToDuplicates = false, //optimizer property
    IsInvariantToOrder = false, //optimizer property
    MaxByteSize = 8000) //maximum size in bytes of persisted value
]
public class Concatenate : IBinarySerialize
{
    /// <summary>
    /// The variable that holds the intermediate result of the concatenation
    /// </summary>
    private StringBuilder intermediateResult;

    /// <summary>
    /// Initialize the internal data structures
    /// </summary>
    public void Init()
    {
        this.intermediateResult = new StringBuilder();
    }

    /// <summary>
    /// Accumulate the next value, not if the value is null
    /// </summary>
    /// <param name="value"></param>
    public void Accumulate(SqlString value)
    {
        if (value.IsNull)
        {
            return;
        }

        this.intermediateResult.Append(value.Value).Append(',');
    }

    /// <summary>
    /// Merge the partially computed aggregate with this aggregate.
    /// </summary>
    /// <param name="other"></param>
    public void Merge(Concatenate other)
    {
        this.intermediateResult.Append(other.intermediateResult);
    }

    /// <summary>
    /// Called at the end of aggregation, to return the results of the aggregation.
    /// </summary>
    /// <returns></returns>
    public SqlString Terminate()
    {
        string output = string.Empty;
        //delete the trailing comma, if any
        if (this.intermediateResult != null
            && this.intermediateResult.Length > 0)
        {
            output = this.intermediateResult.ToString(0, this.intermediateResult.Length - 1);
        }

        return new SqlString(output);
    }

    public void Read(BinaryReader r)
    {
        intermediateResult = new StringBuilder(r.ReadString());
    }

    public void Write(BinaryWriter w)
    {
        w.Write(this.intermediateResult.ToString());
    }
}

[Visual Basic]

Imports System
Imports System.Data
Imports Microsoft.SqlServer.Server
Imports System.Data.SqlTypes
Imports System.IO
Imports System.Text

<Serializable(), SqlUserDefinedAggregate(Format.UserDefined, IsInvariantToNulls:=True, IsInvariantToDuplicates:=False, IsInvariantToOrder:=False, MaxByteSize:=8000)> _
Public Class Concatenate
    Implements IBinarySerialize

    ''' <summary>
    ''' The variable that holds the intermediate result of the concatenation
    ''' </summary>
    Private intermediateResult As StringBuilder

    ''' <summary>
    ''' Initialize the internal data structures
    ''' </summary>
    Public Sub Init()
        Me.intermediateResult = New StringBuilder()
    End Sub

    ''' <summary>
    ''' Accumulate the next value, not if the value is null
    ''' </summary>
    ''' <param name="value"></param>
    Public Sub Accumulate(ByVal value As SqlString)
        If value.IsNull Then
            Return
        End If

        Me.intermediateResult.Append(value.Value).Append(","c)
    End Sub
    ''' <summary>
    ''' Merge the partially computed aggregate with this aggregate.
    ''' </summary>
    ''' <param name="other"></param>
    Public Sub Merge(ByVal other As Concatenate)
        Me.intermediateResult.Append(other.intermediateResult)
    End Sub

    ''' <summary>
    ''' Called at the end of aggregation, to return the results of the aggregation.
    ''' </summary>
    ''' <returns></returns>
    Public Function Terminate() As SqlString
        Dim output As String = String.Empty

        'delete the trailing comma, if any
        If Not (Me.intermediateResult Is Nothing) AndAlso Me.intermediateResult.Length > 0 Then
            output = Me.intermediateResult.ToString(0, Me.intermediateResult.Length - 1)
        End If

        Return New SqlString(output)
    End Function

    Public Sub Read(ByVal r As BinaryReader) Implements IBinarySerialize.Read
        intermediateResult = New StringBuilder(r.ReadString())
    End Sub

    Public Sub Write(ByVal w As BinaryWriter) Implements IBinarySerialize.Write
        w.Write(Me.intermediateResult.ToString())
    End Sub
End Class

Once you compile the code into MyAgg.dll, you can register the aggregate in SQL Server as follows:

CREATE ASSEMBLY MyAgg FROM 'C:\MyAgg.dll'
GO
CREATE AGGREGATE MyAgg (@input nvarchar(200)) RETURNS nvarchar(max)
EXTERNAL NAME MyAgg.Concatenate

Note

On a SQL Server 2005 database with a compatibility level of "80," you cannot create managed user-defined types, stored procedures, functions, aggregates, or triggers. To take advantage of these CLR integration features of SQL Server 2005, you must use the sp_dbcmptlevel (Transact-SQL) stored procedure to set the database compatibility level to "90.".

Note

Managed C++ database objects, such as scalar-valued functions, that have been compiled with the /clr:pure Visual C++ compiler option are not supported for execution in SQL Server 2005 RTM.

As with most aggregates, the bulk of the logic is in the Accumulate method. Here, the string that is passed in as a parameter to the Accumulate method is appended to the StringBuilder object that was initialized in the Init method. Assuming that this is not the first time the Accumulate method has been called, a comma is also appended to the StringBuilder prior to appending the passed-in string. At the conclusion of the computational tasks, the Terminate method is called, which returns the StringBuilder as a string.

For example, consider a table with the following schema:

CREATE TABLE BookAuthors
(
   BookID   int       NOT NULL,
   AuthorName    nvarchar(200) NOT NULL
)

Then insert the following rows:

INSERT BookAuthors VALUES(1, 'Johnson')
INSERT BookAuthors VALUES(2, 'Taylor')
INSERT BookAuthors VALUES(3, 'Steven')
INSERT BookAuthors VALUES(2, 'Mayler')
INSERT BookAuthors VALUES(3, 'Roberts')
INSERT BookAuthors VALUES(3, 'Michaels')

The following query would then produce the following result:

SELECT BookID, dbo.MyAgg(AuthorName)
FROM BookAuthors
GROUP BY BookID
BookID Author Names

1

Johnson

2

Taylor, Mayler

3

Roberts, Michaels, Steven

See Also

Other Resources

CLR User-Defined Aggregates
Database Compatibility Level Option

Help and Information

Getting SQL Server 2005 Assistance

Change History

Release History

14 April 2006
Updated content:
  • Added note on database compatibility level.
  • Added note on managed C++ routines compiled with the /clr:pure option.