select distinct for one column return multiple code example

Example 1: select count distinct multiple columns sql server

SELECT COUNT(DISTINCT (CHECKSUM(DocumentId,DocumentSessionId)) + CHECKSUM(REVERSE(DocumentId),REVERSE(DocumentSessionId)) )
FROM DocumentOutPutItems

Example 2: select distinct on 2 columns

SELECT agent_code, ord_amount, cust_code, ord_num
FROM orders 
WHERE agent_code='A002';