Thanks for the comments!

Of course, I will put FK and I will have a UserID parameter in my
query. So, now the question is, how slower is the first query (if it
is), compared to the second one:

select
Message.id, Message.Subject
from
Messages
inner join UserMessages on (UserMessages.MessageID = Messages.ID)
where
(
(:show_only_unread = 'YES' and UserMessages.IsRead = 'NO') or
(:show_only_unread = 'NO')
)
and
(
UserMessages.UserId = :UserID
)


select
Message.id, Message.Subject
from
Messages
inner join UserMessages on (UserMessages.MessageID = Messages.ID)
where
UserMessages.UserId = :UserID and
UserMessages.IsRead = 'NO'

When parameter "show_only_unread" is "No", there should not be any
performance differences, right? But what about when "show_only_unread"
is "Yes"?

WestSorkin