Flatten association table to multi-value column?

Question

I have a table with just product ID\'s and category ID\'s (products can be in more than one category). How can I flatten the category ID\'s into a product column so I end us

Answer 1

I created an CLR aggregate function that takes a varchar column and returns all its values separated by commas. In other words, it joins several strings into a comma-separated list. I am sure its performance is way better than any T-Sql trick.

As any aggregate function, it can be used in combination with group by. For example:

SELECT id, name, desc, JoinStrings(CONVERT(VARCHAR(20), category_id))
FROM product p
INNER JOIN category_products c ON p.category_id = c.category_id
GROUP BY id, name, desc

Here's the C# code to create the CLR assembly into Sql Server 2008:

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


[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Format.UserDefined, IsInvariantToDuplicates=false, IsInvariantToOrder=false, IsInvariantToNulls=true, MaxByteSize=-1)]
public struct JoinStrings : IBinarySerialize
{
    private char[] sb;
    private int pos;
    public void Init()
    {
        sb = new char[512000];
        pos = 0;
    }

    public void Accumulate(SqlString Value)
    {
        if (Value.IsNull) return;
        char[] src = Value.ToString().ToCharArray();
        Array.Copy(src, 0, sb, pos, src.Length);
        pos += src.Length;
        sb[pos] = ',';
        pos++;
    }

    public void Merge(JoinStrings Group)
    {
        Accumulate(Group.Terminate());
    }

    public SqlString Terminate()
    {
        if (pos <= 0) 
            return new SqlString();
        else
            return new SqlString(new String(sb, 0, pos-1));
    }

    public void Read(System.IO.BinaryReader r)
    {
        this.Init();
        pos = r.ReadInt32();
        r.Read(sb, 0, pos);
    }

    public void Write(System.IO.BinaryWriter w)
    {
        w.Write(pos);
        w.Write(sb, 0, pos);
    }
}

Here's the code to create the function (although deploying from Visual Studio should do it automatically):

CREATE AGGREGATE [dbo].[JoinStrings]
(@s [nvarchar](4000))
RETURNS[nvarchar](max)
EXTERNAL NAME [YouAssemblyName].[JoinStrings]