| Introduction
Many a time, SQL programmers are faced with a requirement to generate report-like resultsets right off of a Transact SQL
query. In most cases, the requirement arises from the fact that there are no sufficient tools or in-house expertise to develop
tools that can extract the data as a resultset and massage the data in the desired display format. Quite often folks are
confused about the potential of breaking relational fundamentals say like First Normal Form or the scalar nature of typed
values. (Talking about 1NF violations in a language like SQL which lacks sufficient domain support, allows NULLs and supports
duplicates is somewhat ironic to begin with, but that is a topic which requires detailed explanations.)
Concatenating column values or expressions from multiple rows are usually best done in a client side application language,
since the string manipulation capabilities of Transact SQL and SQL based DBMSs are somewhat limited. However, you can do these
using different approaches in Transact SQL, but avoiding such methods for long term solutions is your best bet.
A core issue
Even though SQL in general deviates considerably from the relational model, its reliance on certain core aspects of relational
foundations makes SQL functional and powerful. One such core aspect is the set based nature of SQL expressions (well, multi-sets
to be exact, but for the given context let us ignore the issue of duplication). The primary idea is that tables are unordered
and hence the resultsets of any query that does not have an explicit ORDER BY clause is unordered as well. In other words, the rows
in a resultset of a query do not have a prescribed position, unless it is explicitly specified in the query expression.
On the other hand, a concatenated list is an ordered structure. Each element in the list has a specific position. In fact,
concatenation itself is an order-utilizing operation in the sense that values can be prefixed or post fixed to an existing list. So
approaches that are loosely called “concatenating row values”, “aggregate concatenation” etc. would have to make sure that some kind
of an order, either explicit or implicit, should be specified prior to concatenating the row values. If such an ordering criteria
is not provided, the concatenated string would be arbitrary in nature.
Considerations
Generally, requests for row value concatenations often comes in two basic flavors, when the number of rows is known and small (typically
less than 10) and when the number of rows is unknown and potentially large. It may be better to look at each of them separately.
In some cases, all the programmer wants is just the list of values from a set of rows. There is no grouping or logical partitioning
of values like the list of email addresses separated by a semicolon or some such. In such situations, the approaches can be the same
except the join conditions may vary. Minor variations of the examples list on this page illustrate such solutions as well.
For the purpose of this article the Products table from Northwind database is used to illustrate column value concatenations with a
grouping column. Northwind is a sample database in SQL Server 2000 default installations. You can download a copy from from the
Microsoft Downloads
Consider the resultset produced by the following query:
SELECT CategoryId, ProductName
FROM Northwind..Products ;
CategoryId ProductName
----------- ----------------------------------------
1 Chai
1 Chang
...
2 Aniseed Syrup
2 Chef Anton's Cajun Seasoning
...
...
8 Spegesild
(77 row(s) affected)
The goal is to return a resultset with two columns one with the Category Identifier and the other with a concatenated list of all
the Product Names separated by a delimiting character, say a comma for instance.
CategoryId Product List
----------- -----------------------------------------------------------------------------
1 Chai, Chang, Chartreuse verte, Côte de Blaye, ...
2 Aniseed Syrup, Chef Anton's Cajun Seasoning, ...
3 Chocolade, Gumbär Gummibärchen, Maxilaku, ...
4 Camembert Pierrot, Flotemysost, Geitost, Gorgonzola Telino, ...
5 Filo Mix, Gnocchi di nonna Alice, Gustaf's Knäckebröd, ...
6 Alice Mutton, Mishi Kobe Niku, Pâté chinois, ...
7 Longlife Tofu, Manjimup Dried Apples, Rössle Sauerkraut, ...
8 Boston Crab Meat, Carnarvon Tigers, Escargots de Bourgogne, ...
(8 row(s) affected)
Concatenating values when the number of items is small and known upfront
When the number of rows are small and almost known upfront, it is easier to generate the code. One common approach with a small set
of finite rows it the pivoting method. Here is an example where only first four alphabetically sorted product names per categoryid is
retrieved:
SELECT CategoryId,
MAX( CASE seq WHEN 1 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 2 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 3 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 4 THEN ProductName ELSE '' END )
FROM ( SELECT p1.CategoryId, p1.ProductName,
( SELECT COUNT(*)
FROM Northwind.dbo.Products p2
WHERE p2.CategoryId = p1.CategoryId
AND p2.ProductName <= p1.ProductName )
FROM Northwind.dbo.Products p1 ) D ( CategoryId, ProductName, seq )
GROUP BY CategoryId ;
The idea above is to create a expression inside the correlated subquery that produces a rank (seq) based on the product names and
then use it in the outer query. Using common table expressions and the ROW_NUMBER() function, you can re-write this as:
; WITH CTE ( CategoryId, ProductName, seq )
AS ( SELECT p1.CategoryId, p1.ProductName,
ROW_NUMBER() OVER ( PARTITION BY CategoryId ORDER BY ProductName )
FROM Northwind.dbo.Products p1 )
SELECT CategoryId,
MAX( CASE seq WHEN 1 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 2 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 3 THEN ProductName ELSE '' END ) + ', ' +
MAX( CASE seq WHEN 4 THEN ProductName ELSE '' END )
FROM CTE
GROUP BY CategoryId ;
Note that ROW_NUMBER() is a newly introduced feature in SQL 2005. If you are using any previous versions, you will have to use the
subquery approach (You can also use a self-join, to write it a bit differently). Using the recently introduced PIVOT operator, you
can write the above as following :
SELECT CategoryId,
"1" + ', ' + "2" + ', ' + "3" + ', ' + "4" AS Product_List
FROM ( SELECT CategoryId, ProductName,
ROW_NUMBER() OVER (PARTITION BY CategoryId ORDER BY ProductName)
FROM Northwind.dbo.Products ) P ( CategoryId, ProductName, seq )
PIVOT ( MAX( ProductName ) FOR seq IN ( "1", "2", "3", "4" ) ) AS P_ ;
Not only the syntax appears a bit confusing, it does not appear to offer anything functionally beyond the CASE approach above. However,
in rare situations, it could come in handy.
Concatenating values when the number of items is not known
When the number of items that are to be concatenated is not known upfront, the code can become a bit more demanding. The new features
in SQL 2005 make some of the approaches a bit easy. For instance, the recursive common table expressions (CTEs) and the FOR XML PATH('')
syntax makes the server do the hard work behind the concatenation leaving the programmer to deal with the presentation issues. The
examples below make this point obvious.
Recursive CTE methods
The idea behind this method is from a newsgroup posting by Vadim Tropashko similar to the ideas behind generating a materialized path
for hierarchies.
WITH CTE ( CategoryId, product_list, product_name, length )
AS ( SELECT CategoryId, CAST( '' AS VARCHAR(8000) ), CAST( '' AS VARCHAR(8000) ), 0
FROM Northwind..Products
GROUP BY CategoryId
UNION ALL
SELECT p.CategoryId, CAST( product_list +
CASE WHEN length = 0 THEN '' ELSE ', ' END + ProductName AS VARCHAR(8000) ),
CAST( ProductName AS VARCHAR(8000)), length + 1
FROM CTE c
INNER JOIN Northwind..Products p
ON c.CategoryId = p.CategoryId
WHERE p.ProductName > c.product_name )
SELECT CategoryId, product_list
FROM ( SELECT CategoryId, product_list,
RANK() OVER ( PARTITION BY CategoryId ORDER BY length DESC )
FROM CTE ) D ( CategoryId, product_list, rank )
WHERE rank = 1 ;
The CASE in the recursive part of the CTE is used to eliminate the initial comma and you can use RIGHT or the SUBSTRING functions
to substitute it. Also, this may not be the best performing option, however certain additional tuning could be done to make them
suitable for medium sized datasets.
Another approach using recursive common table expressions was sent in by Anub Philip, an Engineer from Sathyam Computers that uses
separate common table expressions for the anchor and recursive parts.
WITH Ranked ( CategoryId, rnk, ProductName )
AS ( SELECT CategoryId,
ROW_NUMBER() OVER( PARTITION BY CategoryId ORDER BY CategoryId ),
CAST( ProductName AS VARCHAR(8000) )
FROM Northwind..Products),
AnchorRanked ( CategoryId, rnk, ProductName )
AS ( SELECT CategoryId, rnk, ProductName
FROM Ranked
WHERE rnk = 1 ),
RecurRanked ( CategoryId, rnk, ProductName )
AS ( SELECT CategoryId, rnk, ProductName
FROM AnchorRanked
UNION ALL
SELECT Ranked.CategoryId, Ranked.rnk,
RecurRanked.ProductName + ', ' + Ranked.ProductName
FROM Ranked
INNER JOIN RecurRanked
ON Ranked.CategoryId = RecurRanked.CategoryId
AND Ranked.rnk = RecurRanked.rnk + 1 )
SELECT CategoryId, MAX( ProductName )
FROM RecurRanked
GROUP BY CategoryId;
On an initial glance, this query may seem a bit expensive in comparison, however the reader is encouraged check the execution plans and
make any additional tweaks as needed.
The blackbox XML methods
An example for string concatenation using FOR XML clause with PATH mode is detailed below. It was initially posted by Eugene Kogan
later became common in public newsgroups.
SELECT p1.CategoryId,
( SELECT ProductName + ','
FROM Northwind.dbo.Products p2
WHERE p2.CategoryId = p1.CategoryId
ORDER BY ProductName
FOR XML PATH('') ) AS Products
FROM Northwind.dbo.Products p1
GROUP BY CategoryId ;
Again, the similar approach originally found in the beta newsgroups, using CROSS APPLY operator.
SELECT DISTINCT CategoryId, ProductNames
FROM Products p1
CROSS APPLY ( SELECT ProductName + ','
FROM Products p2
WHERE p2.CategoryId = p1.CategoryId
ORDER BY ProductName
FOR XML PATH('') ) D ( ProductNames )
You may notice a comma at the end of the concatenated string, which you can remove using a STUFF, SUBSTRING or LEFT function. While the
above methods are deemed reliable by many at the time of writing, there is no guarantee that it will stay that way given the internal
workings and evaluation rules of FOR XML PATH() expression in correlated subqueries are not well documented.
Using Common Language Runtime
Though this article is about approaches using Transact SQL, this section is included due to the popularity of CLR aggregates in SQL 2005. Not only it
empowers the CLR programmer with new options for database development, in some cases, they work at least as well as native Transact
SQL approaches.
If you are familiar with .NET languages, SQL 2005 offers a convenient way to create user defined aggregate functions using C#, VB.NET or
similar languages that is supported by the Common Language Runtime (CLR). Here is an example of a string concatenate aggregate function
written using C#.
using System;
using System.Collections.Generic;
using System.Data.SqlTypes;
using System.IO;
using Microsoft.SqlServer.Server;
[Serializable]
[SqlUserDefinedAggregate(Format.UserDefined, MaxByteSize=8000)]
public struct strconcat : IBinarySerialize{
private List values;
public void Init() {
this.values = new List();
}
public void Accumulate(SqlString value) {
this.values.Add(value.Value);
}
public void Merge(strconcat value) {
this.values.AddRange(value.values.ToArray());
}
public SqlString Terminate() {
return new SqlString(string.Join(", ", this.values.ToArray()));
}
public void Read(BinaryReader r) {
int itemCount = r.ReadInt32();
this.values = new List(itemCount);
for (int i = 0; i <= itemCount - 1; i++) {
this.values.Add(r.ReadString());
}
}
public void Write(BinaryWriter w) {
w.Write(this.values.Count);
foreach (string s in this.values) {
w.Write(s);
}
}
}
Once you build and deploy this assembly on the server, you should be able to execute your concatenation query as:
SELECT CategoryId,
dbo.strconcat(ProductName)
FROM Products
GROUP BY CategoryId ;
If you are a total newbie on CLR languages, and would like to learn more about developing database solutions using CLR languages,
consider starting at Introduction to Common Language Runtime (CLR)
Integration
Scalar UDF with recursion
Recursive functions in t-SQL have a drawback that the maximum nesting level is 32. So this approach is applicable only for smaller
datasets, especially when the number of items within a group, that needs to be concatenated, is less than 32.
CREATE FUNCTION udf_recursive ( @cid INT, @i INT )
RETURNS VARCHAR(8000) AS BEGIN
DECLARE @r VARCHAR(8000), @l VARCHAR(8000)
SELECT @i = @i - 1, @r = ProductName + ', '
FROM Products p1
WHERE CategoryId = @cid
AND @i = ( SELECT COUNT(*) FROM Products p2
WHERE p2.CategoryId = p1.CategoryId
AND p2.ProductName <= p1.ProductName ) ;
IF @i > 0 BEGIN
EXEC @l = dbo.udf_recursive @cid, @i ;
SET @r = @l + @r ;
END
RETURN @r ;
END
This function can be invoked as follows:
SELECT CategoryId,
dbo.udf_recursive( CategoryId, COUNT(ProductName) )
FROM Products
GROUP BY CategoryId ;
Table valued UDF with a WHILE loop
This approach is based on the idea by Linda Wierzbecki where a table variable with three columns is used within a table valued UDF.
The first column represents the group, second represents the currently processing value within a group and the third represents the
concatenated list of values.
CREATE FUNCTION udf_tbl_Concat() RETURNS @t TABLE(
CategoryId INT,
Product VARCHAR(40),
list VARCHAR(8000) )
BEGIN
INSERT @t (CategoryId, Product, list)
SELECT CategoryId, MIN(ProductName), MIN(ProductName)
FROM Products
GROUP BY CategoryId
WHILE ( SELECT COUNT(Product) FROM @t ) > 0 BEGIN
UPDATE t
SET list = list + COALESCE(
( SELECT ', ' + MIN( ProductName )
FROM Products
WHERE Products.CategoryId = t.CategoryId
AND Products.ProductName > t.Product), ''),
Product = ( SELECT MIN(ProductName)
FROM Products
WHERE Products.CategoryId = t.CategoryId
AND Products.ProductName > t.Product )
FROM @t t END
RETURN
END
The usage of the above function can be like:
SELECT CategoryId, list AS Products
FROM udf_tbl_Concat() ;
Dynamic SQL
This approach is a variation of the kludge often known using the nickname as dynamic cross tabulation.
This approach is a variation of the kludge often known using the nickname as dynamic cross tabulation. There is enough literature out
there which demonstrates the drawbacks and implications of using Dynamic SQL. A popular one, at least from Transact SQL programmer’s
perspective, is Erland's Curse and Blessings of Dynamic SQL. The Dynamic SQL
approaches can be developed based on creating a Transact SQL query string based on the number of groups and then use a series of CASE
expressions or ROW_NUMBER() function to pivot the data for concatenation.
DECLARE @r VARCHAR(MAX), @n INT, @i INT
SELECT @i = 1,
@r = 'SELECT CategoryId, ' + CHAR(13),
@n = (SELECT TOP 1 COUNT( ProductName )
FROM Products
GROUP BY CategoryId
ORDER BY COUNT( ProductName ) DESC ) ;
WHILE @i <= @n BEGIN
SET @r = @r +
CASE WHEN @i = 1
THEN 'MAX( CASE Seq WHEN ' + CAST( @i AS VARCHAR ) + '
THEN ProductName
ELSE SPACE(0) END ) + ' + CHAR(13)
WHEN @i = @n
THEN 'MAX( CASE Seq WHEN ' + CAST( @i AS VARCHAR ) + '
THEN '', '' + ProductName
ELSE SPACE(0) END ) ' + CHAR(13)
ELSE 'MAX( CASE Seq WHEN ' + CAST( @i AS VARCHAR ) + '
THEN '', '' + ProductName
ELSE SPACE(0) END ) + ' + CHAR(13)
END ;
SET @i = @i + 1 ;
END
SET @r = @r + '
FROM ( SELECT CategoryId, ProductName,
ROW_NUMBER() OVER ( PARTITION BY CategoryId ORDER BY ProductName )
FROM Products p ) D ( CategoryId, ProductName, Seq )
GROUP BY CategoryId;'
EXEC( @r ) ;
The Cursor approach
The drawbacks of rampant usage of cursors are well-known among the Transact SQL community. Given the fact that they are generally
resource intensive, procedural and inefficient, one should strive to avoid cursors or loop based solutions in general Transact SQL
programming.
DECLARE @tbl TABLE (id INT PRIMARY KEY, list VARCHAR(8000))
SET NOCOUNT ON
DECLARE @c INT, @p VARCHAR(8000), @cNext INT, @pNext VARCHAR(40)
DECLARE c CURSOR FOR
SELECT CategoryId, ProductName
FROM Products
ORDER BY CategoryId, ProductName ;
OPEN c ;
FETCH NEXT FROM c INTO @cNext, @pNext ;
SET @c = @cNext ;
WHILE @@FETCH_STATUS = 0 BEGIN
IF @cNext > @c BEGIN
INSERT @tbl SELECT @c, @p ;
SELECT @p = @PNext, @c = @cNext ;
END ELSE
SET @p = COALESCE(@p + ',', SPACE(0)) + @pNext ;
FETCH NEXT FROM c INTO @cNext, @pNext
END
INSERT @tbl SELECT @c, @p ;
CLOSE c ;
DEALLOCATE c ;
SELECT * FROM @tbl ;
Non-reliable approaches
This section details a couple of notorious methods often publicized by some in public forums. The problem with these methods is that
they rely on the physical implementation model; changes in indexes, statistics etc or even a change of a simple expression in the
SELECT list or ORDER BY clause can change the output. Also these are undocumented, unsupported and unreliable to the point where one
can consistently demonstrate failures. Therefore these methods are not at all recommended for production mode systems.
Scalar UDF with t-SQL update extension
The usage of an expression that involves a column, a variable and an expression in the SET clause in an UPDATE statement rarely appear
intuitive. However, in general, the optimizer often seems to process these values in the order of materialization, either in the internal
work tables or any other storage structures.
CREATE FUNCTION udf_update_concat (@CategoryId INT)
RETURNS VARCHAR(MAX) AS
BEGIN
DECLARE @t TABLE(p VARCHAR(40));
DECLARE @r VARCHAR(MAX) ;
SET @r = SPACE(0) ;
INSERT @t ( p ) SELECT ProductName FROM Products
WHERE CategoryId = @CategoryId ;
IF @@ROWCOUNT > 0
UPDATE @t
SET @r = @r + p + ',' ;
RETURN(@r)
END
Here is how to use this function:
SELECT CategoryId, dbo.udf_update_concat(CategoryId)
FROM Products
GROUP BY CategoryId ;
Again, it is important to consider that lack of physical independence that is being exploited here before using or recommending this as
a usable and meaningful solution.
Scalar UDF with variable concatenation in SELECT
This is an approachpurely dependent on the physical implementation and internal access paths. Before using this approach, make sure to
refer to the relevant knowledgebase article.
CREATE FUNCTION dbo.udf_select_concat ( @c INT )
RETURNS VARCHAR(MAX) AS BEGIN
DECLARE @p VARCHAR(MAX) ;
SET @p = '' ;
SELECT @p = @p + ProductName + ','
FROM Products
WHERE CategoryId = @c ;
RETURN @p
END
And, as for its usage:
SELECT CategoryId, dbo.udf_select_concat( CategoryId )
FROM Products
GROUP BY CategoryId ;
Conclusion
Regardless of how it is used, "aggregate concatenation" of row values in Transact SQL, especially when there is a grouping, is not
a simple routine. Various programming considerations are to be carefully considered to choose one method over another depending on
the situations. The most logical choice would be the availability of a built-in operator with optional configurable parameters that
can do the concatenation of the values depending on the type. Till then, reporting requirements and external data export routines
will have to rely on such Transact SQL programming hacks.
References
Acknowledgements
Umachandar Jayachandran, Linda Wierzbecki, Bruce Margolin, Roy Harvey, Eugene Kogan, Vadim Tropashko, Anub Philip.
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