Hi - Try this.
exists (
(select 1 from doclinks where (ownertable='WORKORDER' and ownerid=:workorderid))
UNION ALL
(select 1 from doclinks where (ownertable in ('SR','INCIDENT','PROBLEM') and ownerid in
(select ticketuid from ticket,relatedrecord where ticketid=recordkey and ticket.class = relatedrecord.class and
relatedrecclass=:woclass and relatedreckey=:wonum and relatedrecsiteid=:siteid))))