Flatten association table to multi-value column? - sql

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 with this:
id | name | desc | categories
1 | test1 | lorem | 1,3,4,23
2 | test2 | ipsom | 4,6,24
It is like I need to loop into a separate table for the categories column. How can I do this or is there a better way?

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]

Answer 2

There's no in-built way to do it in MSSQL.
Simulating group_concat MySQL function in Microsoft SQL Server 2005? has a good description of how to go about implementing a workaround.

Answer 3

I would suggest using a Recursive CTE. I believe that it would be something like this:
select productid, categoryid,
row_number() over (partition by id order by categoryid) as rownum
into #tabletorecurse
from TABLENAME
with finaloutput as
(
select productid as id, name, desc, categoryid as categories, rownum
from #tabletorecurse
join PRODUCTTABLE
on PRODUCTTABLE.id = #tabletorecurse.productid
where rownum = 1
union all
select tr.id, tr.name, tr.desc,
finaloutput.categories + ', ' + tr.categoryid, tr.rownum
from #tabletorecurse as tr
join finaloutput
on finaloutput.rownum + 1 = tr.rownum
and finaloutput.id = tr.productid
)
select id, name, desc, categories
from finaloutput
join
(
select max(rownum) as maxrow, id
from finaloutput
group by id
) as maxvalues
on maxvalues.id = finaloutput.id
and maxvalues.maxrow = finaloutput.rownum

Answer 4

Use a function.
This does a lookup to text so you will need to adapt.
The COALESCE is just to put a ,.
This is from a large scale production application - it works and it fast.
Function was questioned by JustinPony as function is slow
I am hitting some tables of million of records but only returning 100 rows.
The function is only applied to the hundred rows.
usage:
select top 5 sID, ( select [dbo].[JoinMVEnum](docSVsys.sID, '140') ) as [Flag Issue]
from docSVsys
function
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[JoinMVText]
(
#sID int,
#fieldID tinyint
)
RETURNS VARCHAR(MAX)
AS
BEGIN
DECLARE #MVtextList varchar(max)
SELECT #MVtextList = COALESCE(#MVtextList + '; ', '') + docMVtext.value
FROM docMVtext with (nolock)
WHERE docMVtext.sID = #sID and fieldID = #fieldID
RETURN #MVtextList
END
GO