SQL & PL/SQL :: Database Migration Project To Oracle

I am working on database migration project SQL server to oracle, convert this query sql server to oracle basic thing i change but i still getting following error: ORA-01799: A column may not be outer joined to a subquery.


SELECT
h.unique_id,
h.customer_id, h.sequence_id, h.name_first, h.name_last, h.email,
h.tn_work, h.tn_home, h.tn_cell, h.tn_work_do, h.tn_home_do, h.tn_cell_do, h.primary_contact_id,h.ssn, h.created, h.customer_status_id, h.doi, h.dod,
NVL(CASE WHEN h.primary_contact_id = 1 THEN tn_home WHEN h.primary_contact_id = 2 THEN tn_cell
WHEN h.primary_contact_id = 3 THEN tn_work END, '') AS ext_primary_contact, ext_customer_status_desc FROM customer c
INNER JOIN customer_history h
[code]...