T-SQL Challenge: Grouped String Concatenation

Adam Machanic — Fri, 27 Feb 2009 18:22:00 GMT

It's been quite a while since the LIKE vs ? Puzzle, and I feel like it's time for another one. Response was overwhelming last time, and I'm back with a much tougher puzzle and a much bigger prize. So get ready, because I'm going to really make you stretch your brain and your T-SQL skills for this one.

But first, a bit of background. String concatenation is something I've talked about on this blog before, and it is an incredibly popular topic; my post on the subject has gotten more hits than any other single post I've ever done. TechNet blogger Ward Pond also understands the popularity of the topic, having discussed concatenation at least five times on his blog. And as illustrated in this excellent article by Anith Sen, there are a number of methods available to the intrepid SQL Server explorer; the techniques Ward and I show are just the tip of the iceberg.

And while all of that is great, there is a deep and troublesome hidden problem: Nowhere in my post, nor Anith's article, nor Ward's series, will you find a technique that completely solves the concatenation problem. The FOR XML PATH('') method--by far the most popular SQL Server 2005 "trick" I see repeated over and over in these articles and on forums--is a bit limiting. It doesn't help when we need to "group" our string concatenation, i.e. concatenate strings for a number of key values and return multiple rows in the result. And FOR XML PATH('') also leaves us a bit flat if we need to return aggregated data with the concatenated strings or--even more interesting--concatenate multiple different columns in the same output.

Sure, there are ways to solve this problem, but they usually require temp tables, user-defined functions (CLR or otherwise), tables of numbers, cursors, or other adjunct objects. And while some of these solutions are certainly workable they lack the beauty of a single, self-contained solution. In the interest of solving this problem I recently created a challenge for myself: Figure out how to do "grouped" concatenation using nothing more than a single T-SQL statement. No temp tables. No UDFs. No procedural logic.

But rather than do the work all alone and simply post my solution, I've decided to invite you to join me in the quest. Are you up for it?

Here are the rules of the game:

You are to create a single T-SQL statement that concatenates values from the AdventureWorks (note: not AdventureWorks2008) Sales.SalesOrderHeader, Sales.SalesOrderDetail, Production.Product, and Person.Contact tables.
The output should have the following columns, in the following order, and no other columns:

CustomerID: The customer's CustomerID (this is the unique key in the output)
FirstName: The customer's first name
LastName: The customer's last name
OrderCount: Number of orders placed by the customer
TotalDollarAmount: Total dollar amount of all orders placed by the customer (based on the SalesOrderHeader.SubTotal column)
TotalProductQuantity: Total number of items purchased by the customer in all orders (based on SalesOrderDetail.OrderQty)
OrderNumbers: Comma-delimited list containing the order numbers (SalesOrderHeader.SalesOrderNumber) for each of the orders placed by the customer

The numbers within the list should be alphabetized. The list should have neither leading nor trailing commas, and each element in the list should be separated by a single comma with no spaces or other white space beforeor after the comma

ProductNames: Comma-delimited list containing the unique names of all products ordered by the customer in all orders

The names within the list should be alphabetized. The list should have neither leading nor trailing commas, and each element in the list should be separated by a single comma with no spaces or other white space beforeor after the comma

No tables--permanent, temporary, or variable--are to be created. No dynamic SQL is to be used. No user-defined functions, views, or stored procedures are allowed. No variables may be declared. To put it simply, no permanent or temporary objects of any kind, at any scope, are to be explicitly created. No procedural statements of any kind--cursors or control-of-flow--are allowed. This must be a standalone statement in the AdventureWorks database; nothing more and nothing less.
Aside from the previous stipulation, any SQL Server 2005 or 2008 feature is fair game. Documented or not, if it ships with the product and can be used in a standalone T-SQL statement, you can use it. If you do use a version-specific feature, please let me know (especially if it's a SQL Server 2005 feature that's gone in 2008). Bonus points may be given for solutions that work on either version, but I'll make that decision after reviewing the submissions.
Entries will be judged first and foremost on correctness, then on a combination of performance, readability, and ability to apply your technique as a general pattern.

Just to be absolutely clear: If your submission violates the rules, outputs the wrong data, or does not precisely follow the output guidelines listed above, it will be ignored. Last time I spent a lot of energy going back and forth with people helping them get there, and I just don't have the bandwidth to do that again. So double-check your submission before you send it to me.
Make your submission readable or you will lose credit even if performance is amazing. I don't appreciate looking at a mess, and it's good for your career to learn how to write code that others can maintain. Hint: Learn to indent your code properly; lack of indentation is the biggest mistake I see people make with regard to readability.
Take your time. You have two weeks to work on this. I've already come up with three different solutions that have vastly different performance characteristics. Perhaps your first shot isn't the best choice?

The entry deadline is March 16, 2009, midnight GMT. No exceptions.
Submissions should be e-mailed to me, using a .SQL file attachment.

Do not paste your solution into the body of your e-mail
The subject of your e-mail should be "Grouped String Challenge Submission".
E-mail your submissions to [my first name] [at] [this site].
Again, be careful and don't violate these guidelines or your submission will be ignored.

... What's that? You want a prize? Fine, fine ...

The prize, for the best submission, is a full MSDN subscription, valued at around $10,000. How's that for inspiration?

... and that's that! One final note: Please do not post your solution in the comments here or on another blog, before the deadline has been reached! Last time several people did that and it was incredibly annoying both for me and those contestants trying to think through the problem. You won't be doing yourself any favors by trying to mess up the competition.

Once the deadline is reached I will test all of the submissions, tabulate the results, and post back here in early April. I promise, I won't let the thing stagnate for months like I did last time.

Have fun with it, be creative, and feel free to post comments here with any questions you might have. I found this to be a fairly difficult but very interesting exercise and I hope you agree. Enjoy, and I'm looking forward to seeing what you can do!

Adam Machanic : concatenation, puzzle

T-SQL Challenge: Grouped String Concatenation