01476. 00000 - "divisor is equal to zero" Oracle - sql

Question

I have the following code as part of a Script:
ROUND
(
(
(COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END))
/
(COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 OR OFFLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END))
),3
) AS UNIQ_ONLINE_SHOP_RATE
when I run the script I get the 'Divizer is equal to zero' erro.
I ran the denominator and numerator separately which both equal zero so I understand the error.
I have tried NULLiF(,0) as so:
ROUND
(
(
(
COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END) /
nullif((COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 OR OFFLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END)),0)
),3
) AS UNIQ_ONLINE_SHOP_RATE
but then get 'FROM keyword not found where expected error.

Answer 1

Is this your real query? I assume ONLINE_SALES > 0 or OFFLINE_SALES > 0 is the default and ONLINE_SALES = 0 or OFFLINE_SALES = 0 is the exception.
Then in most cases your query would result in
COUNT(DISTINCT CONTACT_KEY) / COUNT(DISTINCT CONTACT_KEY)
which is not so "exciting", i.e. always 1
With some best-guess I would do:
NULLIF(
ROUND(
COUNT(DISTINCT CONTACT_KEY)
/
NULLIF(COUNT(DISTINCT ONLINE_SALES), 0)
, 3)
, 0) AS UNIQ_ONLINE_SHOP_RATE

Answer 2

Here are 3 options (CASE, NULLIF and DECODE), visually simple to understand - use a CTE to calculate both values, while the main query divides them, taking care about 0 as the denominator:
with temp as
(select
COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END) val1,
COUNT(DISTINCT CASE WHEN ONLINE_SALES > 0 OR OFFLINE_SALES > 0 THEN CONTACT_KEY ELSE NULL END) val2
from your_table
)
select
-- using CASE
val1 / case when val2 = 0 then null else val2 end as result_1,
-- using NULLIF
val1 / nullif(val2, 0) as result_2
-- using DECODE
val1 / decode(val2, 0, null, val2) as result_3
from temp;
Shouldn't be difficult to round the result at the end.