You should give alias to your table. So try this:
SELECT A
FROM
(
SELECT A, B FROM TableA
UNION
SELECT A, B FROM TableB
) AS tbl
WHERE B > 'some value'
You should give alias to your table. So try this:
SELECT A
FROM
(
SELECT A, B FROM TableA
UNION
SELECT A, B FROM TableB
) AS tbl
WHERE B > 'some value'