不多說了，說明后面是完整的代碼，用來將字符串型的字段的各行的值拼成一個大字符串，也就是通常所說的Concat

例如有如下表dict

?ID	?NAME	?CATEGORY
?1	RED?	COLOR?
?2	BLUE	COLOR
?3	APPLE?	FRUIT
?4	ORANGE	FRUIT

執(zhí)行SQL語句：select category,dbo.concatenate(name) as names from dict group by category.

得到結(jié)果表如下

?category	?names
?COLOR	REDBLUE?
?FRUIT	?APPLEORANGE

如果覺得需要用逗號或分號或其他任何你想要的分隔符分開，可以修改下面的代碼來實現(xiàn)。

?

在VS2005中，創(chuàng)建一個連接到目標庫的SQL SERVER PROJECT，然后填加一個“聚合”，將下面的代碼復制進去，編譯后，部署即可，然后在SQL SERVER中的“可編程性”“函數(shù)”“聚合函數(shù)”中就可以看到該函數(shù)了。?

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);
??? }

??? /// <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 );
??????? }

??????? 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(diào)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(shù)據(jù)的時候都會調(diào)用一次，并將要處理的數(shù)據(jù)傳給方法。可以在函數(shù)內(nèi)部進行比如比較，合并之類的處理。

本文轉(zhuǎn)自 netcorner 博客園博客，原文鏈接：http://www.cnblogs.com/netcorner/archive/2013/04/13/3017693.html?? ，如需轉(zhuǎn)載請自行聯(lián)系原作者

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