Update table with the same table nested table

Hi,

I am using Oracle 11g.

I have customers table with the below object types as below

CREATE TYPE address_t AS OBJECT (
   street  VARCHAR2(30),
   city    VARCHAR2(20),
   state   CHAR(2),
   zip     CHAR(5) );
/
CREATE TYPE address_tab IS TABLE OF address_t;
/

CREATE TABLE customers (
   custid  NUMBER,
   modified_date date,
   description varchar2(2000),
   del_flag VARCHAR2(10),
   address address_tab )
NESTED TABLE address STORE AS customer_addresses;

INSERT INTO customers VALUES (1,sysdate-10,'abc','N'
            address_tab(
              address_t('101 First', 'Redwood Shores', 'CA', '94065')
            )  

INSERT INTO customers VALUES (2,sysdate-5,'pqr','N'
            address_tab(
              address_t('102 First', 'Redwood Shores', 'CA', '94065')
            )

INSERT INTO customers VALUES (3,sysdate,'mm//mm','N'
            address_tab(
              address_t('103 First', 'Redwood Shores', 'CA', '94065')
            )    

INSERT INTO customers VALUES (4,sysdate-3,'dddd','N'
            address_tab(
              address_t('102 First', 'Redwood Shores', 'CA', '84065')
            )

INSERT INTO customers VALUES (5,sysdate,''yy//mm'','N'
            address_tab(
              address_t('103 First', 'Redwood Shores', 'CA', '84065')
            )

INSERT INTO customers VALUES (6,sysdate,'aa//bb','N')


I need to develop a one time update script as below.

1) Update the "description" column of the customers table with the city and state values from the respective address column(object type)
   as city||'//'||state (in our ex. Redwood Shores//CA)

2) If zip code is same for multiple customers(in ex. 94065 is same for custid=1,2 and 3 customers then,
   update for the MAX modified_date customer(in ex. custid=3 customer) ie desctiption as 'Redwood Shores//CA'

3) Then the other 2 customers ( in ex. custid=1 and 3) mark the del_flag as 'Y' ( as modified date is not latest)

4) If value of address  column does not exists for any customer (in ex. custid=6) then replace the existing description ('//' with '~' )ie in ex. for cust 6 desacription shoud be 'aa~bb'

Can you please suggest how can I achieve this?

Thank you  in advance.