SQL Server 2005 introduced new features for handling and manipulating Xml. Generating Xml via T-SQL is much simpler than it used to be. Parsing Xml is much, much easier, and we have a an Xml data type to use, too. All of that is pretty well documented in the Books Online. So if you need to produce or consume Xml, that's where to look.
But what these posts will be about is using Xml and the Xml features to get some non-Xml things done. Specifically, I'm going to talk about some types of string manipulation that the Xml features make faster and easier.
Generating Comma-Separated Lists (CSV Strings) Sometimes, when you're summarizing data, you want the values in a column to be aggregated into a comma-separated list. An example of this might be a list of your biggest customers, with the trade associations or buying groups they belong to listed off to one side.
As you know, there is no built-in aggregate to do this in SQL Server. Microsoft (and, I believe, others) have published a
SQL CLR aggregate function that will do this for you.
But, thanks to the Xml features, this can be also be done with T-SQL code, without publishing assemblies or writing custom functions. I’m going to demonstrate it with a series of CTEs, so that each step in the transformation is separate, but you can combine all of them into one expression if you like.
Let’s suppose that we are querying the AdventureWorks2008 database, and we want a list of stores with a list of contacts for each store in the same row, in a comma-separated list.
We start by querying the Sales.Store table, and include a subquery of the Person.Contact table, using the Sales.StoreContact table to link between stores and contacts. Our subquery is going to return the names of the contacts as an Xml fragment.
select CustomerId
, Name
, (
Select ContactName = FirstName + ' ' + ISNULL(MiddleName + ' ','') + LastName
from Person.Contact pc
inner join
Sales.StoreContact ssc
on pc.ContactID = ssc.ContactID
where ssc.CustomerID = ss.CustomerID
for xml auto, elements, type
) as ContactList1
from Sales.Store ss
This looks like this:
Note that we’re only returning one element in our fragments: the Contact Name element. This is the only text that the fragment contains.
Our next step transforms the ContactList1 Xml field into a space-delimited list of names. Because our data has spaces, we’re going to alter our subquery a little to replace the spaces in the data with a character unlikely to appear in a name. You’ll see why in a minute.
With A
as (
Select ContactName = FirstName + ' ' + ISNULL(MiddleName + ' ','') + LastName
, ssc.CustomerID
from Person.Contact pc
inner join
Sales.StoreContact ssc
on pc.ContactID = ssc.ContactID
), B
as (
select CustomerId
, Name
, (
select ContactName = REPLACE(A.ContactName, ' ','|')
from A
where CustomerID = ss.CustomerID
for xml auto, elements, type
) as ContactList1
from Sales.Store ss
)
select B.*
, ContactList2 = B.ContactList1.query('data(*)')
from B
This produces results like this:
To convert this to a comma-separated list, we just need to do a conversion and some REPLACE statements:
With A
as (
Select ContactName = FirstName + ' ' + ISNULL(MiddleName + ' ','') + LastName
, ssc.CustomerID
from Person.Contact pc
inner join
Sales.StoreContact ssc
on pc.ContactID = ssc.ContactID
)
, B
as (
select CustomerId
, Name
, (
select ContactName = REPLACE(A.ContactName, ' ','|')
from A
where CustomerID = ss.CustomerID
for xml auto, elements, type
) as ContactList1
from Sales.Store ss
)
, C
as (
select B.*
, ContactList2 = B.ContactList1.query('data(*)')
from B
)
select C.CustomerID
, C.Name
, ContactList = REPLACE(
REPLACE(
CONVERT(varchar(max), C.ContactList2)
, ' ' -- convert spaces ...
, ', ' -- to commas (followed by spaces, if youlike)
)
, '|' -- replace the pipe char ...
, ' ' -- with the original spaces!
)
from C
… and the result looks like this:
Voilà: inline comma-separated lists!
Labels: csv, sql, xml
