[sql] SQL Query - Concatenating Results into One String

I have a sql function that includes this code:

DECLARE @CodeNameString varchar(100)

SELECT CodeName FROM AccountCodes ORDER BY Sort

I need to concatenate all results from the select query into CodeNameString.

Obviously a FOREACH loop in C# code would do this, but how do I do it in SQL?

For SQL Server 2005 and above use Coalesce for nulls and I am using Cast or Convert if there are numeric values -

declare @CodeNameString  nvarchar(max)
select  @CodeNameString = COALESCE(@CodeNameString + ',', '')  + Cast(CodeName as varchar) from AccountCodes  ORDER BY Sort
select  @CodeNameString

@AlexanderMP's answer is correct, but you can also consider handling nulls with coalesce:

declare @CodeNameString  nvarchar(max)
set @CodeNameString = null
SELECT @CodeNameString = Coalesce(@CodeNameString + ', ', '') + cast(CodeName as varchar) from AccountCodes  
select @CodeNameString

Here is another real life example that works fine at least with 2008 release (and later).

This is the original query which uses simple max() to get at least one of the values:

SELECT option_name, Field_M3_name, max(Option_value) AS "Option value", max(Sorting) AS "Sorted"
FROM Value_list group by Option_name, Field_M3_name
ORDER BY option_name, Field_M3_name

Improved version, where the main improvement is that we show all values comma separated:

SELECT from1.keys, from1.option_name, from1.Field_M3_name,

 Stuff((SELECT DISTINCT ', ' + [Option_value] FROM Value_list from2
  WHERE COALESCE(from2.Option_name,'') + '|' + COALESCE(from2.Field_M3_name,'') = from1.keys FOR XML PATH(''),TYPE)
  .value('text()[1]','nvarchar(max)'),1,2,N'') AS "Option values",

 Stuff((SELECT DISTINCT ', ' + CAST([Sorting] AS VARCHAR) FROM Value_list from2
  WHERE COALESCE(from2.Option_name,'') + '|' + COALESCE(from2.Field_M3_name,'') = from1.keys FOR XML PATH(''),TYPE)
  .value('text()[1]','nvarchar(max)'),1,2,N'') AS "Sorting"

FROM ((SELECT DISTINCT COALESCE(Option_name,'') + '|' + COALESCE(Field_M3_name,'') AS keys, Option_name, Field_M3_name FROM Value_list)
-- WHERE
) from1
ORDER BY keys

Note that we have solved all possible NULL case issues that I can think of and also we fixed an error that we got for numeric values (field Sorting).

from msdn Do not use a variable in a SELECT statement to concatenate values (that is, to compute aggregate values). Unexpected query results may occur. This is because all expressions in the SELECT list (including assignments) are not guaranteed to be executed exactly once for each output row

The above seems to say that concatenation as done above is not valid as the assignment might be done more times than there are rows returned by the select

DECLARE @CodeNameString varchar(max)
SET @CodeNameString=''

SELECT @CodeNameString=@CodeNameString+CodeName FROM AccountCodes ORDER BY Sort
SELECT @CodeNameString