embedding conditional filter into a concatentated query subject

[ry1633]

is there a way to embed a condition into a concatenated query subject?   I has a concatenated site address that I would like to put a conditional filter in it; so it only shows site addresses related to a certain site type field for instance.    Like only show site addresses for site type = '999' or something like that.

Here's the definition for my site address query subject.

[Database Layer].[site_table].[ADDRESS_1] || ', ' ||[Database Layer].[site_table].[CITY] || ' ' || [Database Layer].[site_table].[STATE] || ' ' || [Database Layer].[site_table].[ZIP]

[bus_pass_man]

Something like this?
If
(
site addresses for site type = '999'
)
then
([Database Layer].[site_table].[ADDRESS_1] || ', ' ||[Database Layer].[site_table].[CITY] || ' ' || [Database Layer].[site_table].[STATE] || ' ' || [Database Layer].[site_table].[ZIP]
)
else
(
whatever you want to put in for those records where it isn't 999
)