How to filter a list of values ​​based on information that is in another field on the screen?

Summary:

I'd like to leave the list of values ​​in the "Case" field dependent on the information in the "Customer Site" field. Is it possible to do that?

Content (required):

SELECT ppav.segment1

 FROM hz_parties hp

    , hz_cust_accounts hcaa

    , hz_cust_acct_sites_all hcas

    , hz_party_sites hps

    , hz_locations hl

    , hz_cust_site_uses_all hcsu

    , pjf_projects_all_vl ppav

 WHERE 1=1

  AND hp.party_id                 = hcaa.party_id

  AND hcaa.cust_account_id        = hcas.cust_account_id

  AND hcas.party_site_id          = hps.party_site_id

  AND hps.location_id             = hl.location_id

  AND hcas.cust_acct_site_id      = hcsu.cust_acct_site_id