The UNION keyword is used to combine the result-set of two or more SELECT statements. For example:

SELECT tickets.id
FROM (
     (SELECT tickets.id FROM tickets WHERE tickets.id > 0 AND tickets.id < 5)
     UNION DISTINCT
     (SELECT tickets.id FROM tickets WHERE tickets.id > 5 AND tickets.id < 10)
) AS 'tickets'
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UNION needs to be embedded inside FROM (technically making it a sub-query).