SQL & PL/SQL

Hi,

I'm writing a piece of code that works fine until I place the distinct function on the third line, and then I get the oracle error: ORA-00936: Missing Expression.

Does anyone understand where I'm going wrong?

      select
            customer_id,
            *distinct* substr(old_system_id, instr(old_system_id, ':') +1, 4),
            substr(old_system_id, 1, instr(old_system_id,':')-1),
            old_system_id
      from
            ph1_ca_load
      inner join
            ph1_site_map_load
      on
            substr(old_system_id, 1, instr(old_system_id,':')-1) = site_code
      inner join
            customer
      on
            name = billing_customer
;