Using UNION ALL in STUFF / XML Path

Question

Msg 1086, Level 15, State 1, Line 20 The FOR XML clause is invalid in views, inline functions, derived tables, and subqueries when they contain a set operator. To work around, wrap the SELECT containing a set operator using derived table syntax and apply FOR XML on top of it.

I get this error when i run this:

SELECT
    STUFF((
    SELECT 1
    UNION ALL
    SELECT 2
    FOR XML PATH('')
    ),1,0,'') [COLUMN]

works fine when i run this (without Union ALL)

SELECT
    STUFF((
    SELECT 1
    FOR XML PATH('')
    ),1,0,'') [COLUMN]

Any suggestions why UNION ALL Doesn't work, or how to get it to work inside the STUFF()?

Solution

There's a simple workaround for that, you should wrap your union query(or any derived table for that matter) with another select. Do this and then continue the syntax normally:

select * from
(
SELECT 1 as I
UNION ALL
SELECT 2 as J
) as K

Something like this is what you're searching for:

SELECT  STUFF((
    select * from(

    SELECT * from dbo.Table1 as I
    UNION ALL
    SELECT * from dbo.Table2 as j
    ) as k
    FOR XML PATH('')
    ),1,0,'')

I checked and it works flawlessly

Comments

[alzaimar]

What are the aliases I and J for?

[Gaspa79]

I believe they're not needed, I put them just in case. If you put them you can refer to table dbo.Table1 as j in future references. However, the k IS NEEDED because if it weren't there, there'd be no way to refer to the columns of that subquery.

[Cee McSharpface]

how, why, where, when, what? (is this documented anywhere? obviously not on MSDN) - is it version specific?