SQL Server中的聚合，常用的比如max,count之類。 我們現在也可以在SQLCLR里創(chuàng)建自定義的聚合。Visual Studio 2005中提供的聚合模板是一個結構，標注了[Serializable]，[SqlUserDefinedAggregate]標簽，這將讓SQLCLR知道這是一個聚合函數。
看一段代碼，這段代碼來自SQL Server 2005聯機叢書，本來自己想寫一段，但突然公司有些事要做，沒時間了。示例代碼作用是合并同一部書(ID相同)的作者。
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());
????}
}這里有幾個比較重要的方法：Terminate，這個方法是聚合最后調用的方法，它返回最后的值。可以是SQL Server的任何標量。；Accumulate，聚合每處理一行數據的時候都會調用一次，并將要處理的數據傳給方法。可以在函數內部進行比如比較，合并之類的處理。；
CREATE?TABLE?BookAuthors
(
???BookID???int???????NOT?NULL,
???AuthorName????nvarchar(200)?NOT?NULL
)

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')

SELECT?BookID,?dbo.MyAgg(AuthorName)
FROM?BookAuthors
GROUP?BY?BookID
結果如下
BookID Author Names

1	Johnson
2	Taylor, Mayler
3	Roberts, Michaels, Steven

Microsoft SQL Server Management Studio為我們提供了數據庫內對象的集中管理功能，前面幾篇創(chuàng)建的SQLCLR對象，都可以在數據庫的可編程性下相應模塊里找到。

這一系列到此就算是結束了，謝謝大家。

轉載于:https://www.cnblogs.com/wayne-ivan/archive/2007/06/19/788555.html

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