SELECT * FROM
(
    SELECT  a
    FROM    b
) `firstQuery`

UNION

SELECT * FROM
(
    SELECT  b
    FROM    c
) `secondQuery`

SELECT * FROM
(
    SELECT  a
    FROM    b
) `firstQuery`
UNION
SELECT * FROM
(
    SELECT  b
    FROM    c
    WHERE NOT EXISTS
    (
        SELECT * FROM `firstQuery`
    )
) `secondQuery`