The FOR XML and FOR JSON clauses are 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 or common table expression or view and apply FOR XML or FOR JSON on top of it.

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

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