Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (PARTY_NAME, location_id, party_site_name, Email_Address) AS
SELECT 'Party 1', 1006746009, 'CUSTOM PRODUCTS', 'newton@testemail.net' FROM DUAL UNION ALL
SELECT 'Party 1', 1006746009, 'CUSTOM PRODUCTS', 'lorne@testemail.net' FROM DUAL UNION ALL
SELECT 'Party 2', 1006746010, 'PAINT COMPANY', 'tgliwell@testemail.com' FROM DUAL UNION ALL
SELECT 'Party 2', 1006746010, 'PAINT COMPANY', 'APAL@testemail.com' FROM DUAL UNION ALL
SELECT 'Party 3', 1006746011, 'ADVANCED MATERIALS', 'invo@testemail.com' FROM DUAL UNION ALL
SELECT 'Party 4', 1006746012, 'ADVANCED PRODUCTS', 'accpayable@testemail.com' FROM DUAL;
6 rows affected

SELECT party_name
      ,location_id "Bill To Number"
      ,party_site_name "Customer Bill To Name"
      ,email_address "Customer Email Address"
      ,CHR(
         64
         + ROW_NUMBER() OVER (
             PARTITION BY party_name, location_id, party_site_name
             ORDER BY email_address
           )
       ) AS Dummy
FROM   table_name


PARTY_NAME
Bill To Number
Customer Bill To Name
Customer Email Address
DUMMY


Party 1
1006746009
CUSTOM PRODUCTS
lorne@testemail.net
A

Party 1
1006746009
CUSTOM PRODUCTS
newton@testemail.net
B

Party 2
1006746010
PAINT COMPANY
APAL@testemail.com
A

Party 2
1006746010
PAINT COMPANY
tgliwell@testemail.com
B

Party 3
1006746011
ADVANCED MATERIALS
invo@testemail.com
A

Party 4
1006746012
ADVANCED PRODUCTS
accpayable@testemail.com
A

PARTY_NAME	Bill To Number	Customer Bill To Name	Customer Email Address	DUMMY
Party 1	1006746009	CUSTOM PRODUCTS	lorne@testemail.net	A
Party 1	1006746009	CUSTOM PRODUCTS	newton@testemail.net	B
Party 2	1006746010	PAINT COMPANY	APAL@testemail.com	A
Party 2	1006746010	PAINT COMPANY	tgliwell@testemail.com	B
Party 3	1006746011	ADVANCED MATERIALS	invo@testemail.com	A
Party 4	1006746012	ADVANCED PRODUCTS	accpayable@testemail.com	A