R12 - How to link GL data to the subledger data or vice versa

gl_je_lines (je_header_id, je_line_num)                -> gl_import_references (je_header_id, je_line_num)

gl_import_references (gl_sl_link_table, gl_sl_link_id) -> xla_ae_lines (gl_sl_link_table, gl_sl_link_id)

xla_ae_lines (applicaiton_id, ae_header_id)            -> xla_ae_headers (application_id, ae_header_id)

xla_ae_headers (application_id, event_id)              -> xla_events (application_id, event_id)

xla_events (application_id, entity_id)                 -> xla.xla_transaction_entities (application_id, entity_id)

xla.xla_transaction_entities (source_id_int_1, etc) after filtering by application_id, entity_code and ledger_id     -> subledger's table(its key columns mentioned in xla_entity_id_mappings) for that ledger_id

For Example:
xla.xla_transaction_entities (source_id_int_1) filtered by application_id 200, entity_code AP_INVOICES and ledger_id -> ap_invoices_all (invoice_id) for that set_of_books_id.
xla.xla_transaction_entities (source_id_int_1) filtered by application_id 200, entity_code AP_PAYMENTS and ledger_id -> ap_checks_all (check_id) for that set_of_books_id.

xla.xla_transaction_entities (source_id_int_1) filtered by application_id 222, entity_code TRANSACTIONS and ledger_id -> ra_customer_trx_all (customer_trx_id) for that set_of_books_id.

Note:
a) There is an index on xla.xla_transaction_entities on the following columns.
application_id
entity_code
ledger_id
nvl(source_id_int_1,-99)
nvl(source_id_int_2,-99)
nvl(source_id_int_3,-99)
nvl(source_id_int_4,-99)
nvl(source_id_char_1,' ')
...
nvl(source_id_char_4,' ')

b) Use application_id filter wherever it is possible, as above mentioned XLA tables are partitioned by that.

Query for Ledger, OU, Legal Enity

Query for Ledger, OU, Legal Enity, balancing segment:

SELECT hrl.country, hroutl_bg.NAME bg, hroutl_bg.organization_id,
       lep.legal_entity_id, lep.NAME legal_entity,
       hroutl_ou.NAME ou_name, hroutl_ou.organization_id org_id,
       hrl.location_id,
       hrl.location_code,
       glev.FLEX_SEGMENT_VALUE
  FROM xle_entity_profiles lep,
       xle_registrations reg,
       hr_locations_all hrl,
       hz_parties hzp,
       fnd_territories_vl ter,
       hr_operating_units hro,
       hr_all_organization_units   tl hroutl_bg,
       hr_all_organization_units_tl hroutl_ou,
       hr_organization_units gloperatingunitseo,
       gl_legal_entities_bsvs glev
 WHERE lep.transacting_entity_flag = 'Y'
   AND lep.party_id = hzp.party_id
   AND lep.legal_entity_id = reg.source_id
   AND reg.source_table = 'XLE_ENTITY_PROFILES'
   AND hrl.location_id = reg.location_id
   AND reg.identifying_flag = 'Y'
   AND ter.territory_code = hrl.country
   AND lep.legal_entity_id = hro.default_legal_context_id
   AND gloperatingunitseo.organization_id = hro.organization_id
   AND hroutl_bg.organization_id = hro.business_group_id
   AND hroutl_ou.organization_id = hro.organization_id
   AND glev.legal_entity_id = lep.legal_entity_id