How to count unique rows in Oracle

It depends on what you are trying to accomplish. To get a count of the distinct rows by specific column, so that you know what data exists, and how many of that distinct data there are:

SELECT DISTINCT 

A_CODE, COUNT(*) 

FROM MY_ARCHV

GROUP BY A_CODE

--This informs me there are 93 unique codes, and how many of each of those codes there are.

Another method

--How to count how many of a type value exists in an oracle table:
select A_CDE, --the value you need to count
count(*) as numInstances --how many of each value
from A_ARCH -- the table where it resides
group by A_CDE -- sorting method

Either way, you get something that looks like this:

A_CODE  Count(*)

1603    32

1600    2

1605    14

How about

SELECT COUNT(*) 
FROM (SELECT DISTINCT * FROM Table)