How to group-concatenate multiple columns? - sql

Question

Assume this table:
PruchaseID | Customer | Product | Method
-----------|----------|----------|--------
1 | John | Computer | Credit
2 | John | Mouse | Cash
3 | Will | Computer | Credit
4 | Will | Mouse | Cash
5 | Will | Speaker | Cash
6 | Todd | Computer | Credit
I want to generate a report on each customer of what they bought, and their payment methods.
But I want that report to be one row per customer, such as:
Customer | Products | Methods
---------|--------------------------|--------------
John | Computer, Mouse | Credit, Cash
Will | Computer, Mouse, Speaker | Credit, Cash
Todd | Computer | Credit
What I've found so far is to group-concatenate using the XML PATH method, such as:
SELECT
p.Customer,
STUFF(
SELECT ', ' + xp.Product
FROM Purchases xp
WHERE xp.Customer = p.Customer
FOR XML PATH('')), 1, 1, '') AS Products,
STUFF(
SELECT ', ' + xp.Method
FROM Purchases xp
WHERE xp.Customer = p.Customer
FOR XML PATH('')), 1, 1, '') AS Methods
FROM Purchases
This gives me the result, but my concern is the speed of this.
At first glance there are three different selects going on here, two would each multiply by the number of rows Purchases has. Eventually this would slow down expenentially.
So, is there a way to do this with better performance?
I want to add even more columns to aggregate, should I do this STUFF() block for every column? That doesn't sound fast enough for me.
Siggestions?

Answer 1

Just an idea:
DECLARE #t TABLE (
Customer VARCHAR(50),
Product VARCHAR(50),
Method VARCHAR(50),
INDEX ix CLUSTERED (Customer)
)
INSERT INTO #t (Customer, Product, Method)
VALUES
('John', 'Computer', 'Credit'),
('John', 'Mouse', 'Cash'),
('Will', 'Computer', 'Credit'),
('Will', 'Mouse', 'Cash'),
('Will', 'Speaker', 'Cash'),
('Todd', 'Computer', 'Credit')
SELECT t.Customer
, STUFF(CAST(x.query('a/text()') AS NVARCHAR(MAX)), 1, 2, '')
, STUFF(CAST(x.query('b/text()') AS NVARCHAR(MAX)), 1, 2, '')
FROM (
SELECT DISTINCT Customer
FROM #t
) t
OUTER APPLY (
SELECT DISTINCT [a] = CASE WHEN id = 'a' THEN ', ' + val END
, [b] = CASE WHEN id = 'b' THEN ', ' + val END
FROM #t t2
CROSS APPLY (
VALUES ('a', t2.Product)
, ('b', t2.Method)
) t3 (id, val)
WHERE t2.Customer = t.Customer
FOR XML PATH(''), TYPE
) t2 (x)
Output:
Customer Product Method
---------- -------------------------- ------------------
John Computer, Mouse Cash, Credit
Todd Computer Credit
Will Computer, Mouse, Speaker Cash, Credit
Another idea with more performance benefits:
IF OBJECT_ID('tempdb.dbo.#EntityValues') IS NOT NULL
DROP TABLE #EntityValues
DECLARE #Values1 VARCHAR(MAX)
, #Values2 VARCHAR(MAX)
SELECT Customer
, Product
, Method
, RowNum = ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY 1/0)
, Values1 = CAST(NULL AS VARCHAR(MAX))
, Values2 = CAST(NULL AS VARCHAR(MAX))
INTO #EntityValues
FROM #t
UPDATE #EntityValues
SET
#Values1 = Values1 =
CASE WHEN RowNum = 1
THEN Product
ELSE #Values1 + ', ' + Product
END
, #Values2 = Values2 =
CASE WHEN RowNum = 1
THEN Method
ELSE #Values2 + ', ' + Method
END
SELECT Customer
, Values1 = MAX(Values1)
, Values2 = MAX(Values2)
FROM #EntityValues
GROUP BY Customer
But with some limitations:
Customer Values1 Values2
------------- ----------------------------- ----------------------
John Computer, Mouse Credit, Cash
Todd Computer Credit
Will Computer, Mouse, Speaker Credit, Cash, Cash
Also check my old post about string aggregation:
http://www.codeproject.com/Articles/691102/String-Aggregation-in-the-World-of-SQL-Server

Answer 2

Another solution is the CLR method for group concatenation #aaron bertrand has done a performance comparison on this here.
If you can deploy CLR then download the script from https://orlando-colamatteo.github.io/ms-sql-server-group-concat-sqlclr/ which is free.
and all details are there in the documentation.
Your query will just change into like this
SELECT Customer,dbo.GROUP_CONCAT(product),dbo.GROUP_CONCAT(method)
FROM Purchases
GROUP BY Customer
This query is short, easy to remember and use, XML method also does the job but remembering the code is a bit difficult(atleast for me) and creeps in the problem like XML entitization which can be solved sure and some pitfalls also described in his blog.
Also from a performance view point using .query is time consuming I had the same issues with performance. I hope you can find this question I raised here in https://dba.stackexchange.com/questions/125771/multiple-column-concatenation
check the version 2 given by kenneth fisher a nested xml concatenation method or a unpivot /pivot method suggested by spaggettidba.

Answer 3

This is one of the use cases for recursive CTEs (Common Table Expressions). You can learn more here https://technet.microsoft.com/en-us/library/ms190766(v=sql.105).aspx
;
WITH CTE1 (PurchaseID, Customer, Product, Method, RowID)
AS
(
SELECT
PurchaseID, Customer, Product, Method,
ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY Customer)
FROM
#tbl
/* This table holds source data. I ommited declaring and inserting
data into it because that's not important. */
)
, CTE2 (PurchaseID, Customer, Product, Method, RowID)
AS
(
SELECT
PurchaseID, Customer,
CONVERT(VARCHAR(MAX), Product),
CONVERT(VARCHAR(MAX), Method),
1
FROM
CTE1
WHERE
RowID = 1
UNION ALL
SELECT
CTE2.PurchaseID, CTE2.Customer,
CONVERT(VARCHAR(MAX), CTE2.Product + ',' + CTE1.Product),
CONVERT(VARCHAR(MAX), CTE2.Method + ',' + CTE1.Method),
CTE2.RowID + 1
FROM
CTE2 INNER JOIN CTE1
ON CTE2.Customer = CTE1.Customer
AND CTE2.RowID + 1 = CTE1.RowID
)
SELECT Customer, MAX(Product) AS Products, MAX(Method) AS Methods
FROM CTE2
GROUP BY Customer
Output:
Customer Products Methods
John Computer,Mouse Credit,Cash
Todd Computer Credit
Will Computer,Mouse,Speaker Credit,Cash,Cash