通过

调用 CLR 用户定义的聚合函数

  • 项目

适用范围：SQL Server

在 Transact-SQL SELECT 语句中，可以调用公共语言运行时（CLR）用户定义的聚合，但受应用于系统聚合函数的所有规则的约束。

应遵循下列附加规则：

  • 当前用户必须对用户定义的聚合具有 EXECUTE 权限。

  • 必须以 <schema_name>形式使用由两部分组成的名称调用用户定义的聚合。<udagg_name>。

  • 用户定义的聚合的参数类型必须与 CREATE AGGREGATE 语句中定义的聚合 input_type 匹配或隐式转换为聚合的 input_type。

  • 用户定义的聚合的返回类型必须与 CREATE AGGREGATE 语句中的 return_type 匹配。

例子

一个。 用户定义的聚合串联字符串值

以下代码是用户定义的聚合函数的示例，该函数连接从表中的列获取的一组字符串值：

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>
    public 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());
    }
}

将代码编译为 MyAgg.dll后，可以在 SQL Server 中注册聚合，如下所示：

CREATE ASSEMBLY MyAgg
    FROM 'C:\MyAgg.dll';
GO

CREATE AGGREGATE MyAgg(@input NVARCHAR (200))
    RETURNS NVARCHAR (MAX)
    EXTERNAL NAME MyAgg.Concatenate;

注意

SQL Server 中不支持使用 /clr:pure 编译器选项编译的可视化C++数据库对象（如标量值函数）。

与大多数聚合一样，大部分逻辑位于 Accumulate 方法中。 在这里，作为参数传入 Accumulate 方法的字符串将追加到在 Init 方法中初始化的 StringBuilder 对象。 假设尚未调用 Accumulate 方法，在追加传入字符串之前，还会将逗号追加到 StringBuilder。 在计算任务结束时，将调用 Terminate 方法，该方法将 StringBuilder 作为字符串返回。

例如，请考虑具有以下架构的表：

CREATE TABLE BookAuthors
(
    BookID INT NOT NULL,
    AuthorName NVARCHAR (200) NOT NULL
);

然后插入以下行：

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

随后，以下查询将生成以下结果：

SELECT BookID, dbo.MyAgg(AuthorName)
FROM BookAuthors
GROUP BY BookID;
BookID Author Names
1 Johnson
2 Taylor, Mayler
3 Roberts, Michaels, Steven

B. 具有两个参数的用户定义聚合

以下示例显示了一个聚合，该聚合在 Accumulate 方法上具有两个参数。

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedAggregate(
    Format.Native,
    IsInvariantToDuplicates = false,
    IsInvariantToNulls = true,
    IsInvariantToOrder = true,
    IsNullIfEmpty = true,
    Name = "WeightedAvg")]
public struct WeightedAvg
{
    /// <summary>
    /// The variable that holds the intermediate sum of all values multiplied by their weight
    /// </summary>
    private long sum;

    /// <summary>
    /// The variable that holds the intermediate sum of all weights
    /// </summary>
    private int count;

    /// <summary>
    /// Initialize the internal data structures
    /// </summary>
    public void Init()
    {
        sum = 0;
        count = 0;
    }

    /// <summary>
    /// Accumulate the next value, not if the value is null
    /// </summary>
    /// <param name="Value">Next value to be aggregated</param>
    /// <param name="Weight">The weight of the value passed to Value parameter</param>
    public void Accumulate(SqlInt32 Value, SqlInt32 Weight)
    {
        if (!Value.IsNull && !Weight.IsNull)
        {
            sum += (long)Value * (long)Weight;
            count += (int)Weight;
        }
    }

    /// <summary>
    /// Merge the partially computed aggregate with this aggregate
    /// </summary>
    /// <param name="Group">The other partial results to be merged</param>
    public void Merge(WeightedAvg Group)
    {
        sum += Group.sum;
        count += Group.count;
    }

    /// <summary>
    /// Called at the end of aggregation, to return the results of the aggregation.
    /// </summary>
    /// <returns>The weighted average of all inputed values</returns>
    public SqlInt32 Terminate()
    {
        if (count > 0)
        {
            int value = (int)(sum / count);
            return new SqlInt32(value);
        }
        else
        {
            return SqlInt32.Null;
        }
    }
}

编译 C# 或 Visual Basic .NET 源代码后，运行以下 Transact-SQL。 此脚本假设相应的 DLL 名为 WghtAvg.dll，并且位于 C 驱动器的根目录下， 还假设数据库名为 test。

USE test;
GO

-- EXECUTE sp_configure 'clr enabled', 1;
-- RECONFIGURE WITH OVERRIDE;
-- GO
IF EXISTS (SELECT name
           FROM systypes
           WHERE name = 'MyTableType')
    DROP TYPE MyTableType;
GO

IF EXISTS (SELECT name
           FROM sysobjects
           WHERE name = 'WeightedAvg')
    DROP AGGREGATE WeightedAvg;
GO

IF EXISTS (SELECT name
           FROM sys.assemblies
           WHERE name = 'MyClrCode')
    DROP ASSEMBLY MyClrCode;
GO

CREATE ASSEMBLY MyClrCode
    FROM 'C:\WghtAvg.dll';
GO

CREATE AGGREGATE WeightedAvg(@value INT, @weight INT)
    RETURNS INT
    EXTERNAL NAME MyClrCode.WeightedAvg;
GO

CREATE TYPE MyTableType AS TABLE (
    ItemValue INT,
    ItemWeight INT);
GO

DECLARE @myTable AS MyTableType;

INSERT INTO @myTable
VALUES (1, 4),
(6, 1);

SELECT dbo.WeightedAvg(ItemValue, ItemWeight)
FROM @myTable;
GO

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