How to apply a NTILE function to a SQL column - sql

Question

So im trying to establish quantiles over the number of attentions received by some customers, but leaving NULLs and 0 out of the sample and placing them in the 0 and null quantil before applying NTILES function, I'm using this function:
CREATE FUNCTION dbo.RPS_Ntiles (#data int,#percentil int)
RETURNS TABLE
AS
RETURN
SELECT
NTILE(#percentil) OVER(ORDER BY #data DESC) as QUINTIL
WHERE #data > 0
UNION
SELECT
CASE
WHEN CONVERT(int,#data) = 0 THEN 0
WHEN #data is NULL THEN NULL
END AS QUINTIL
WHERE #data = 0 or #data = NULL
Then I'm applying it using CROSS APPLY
SELECT t.QUINTIL,D.[atenciones]
FROM table as D
CROSS APPLY [dbo].[RPS_NTiles](CONVERT(int,D.[atenciones]),5) as t
ORDER BY D.[atenciones] DESC
The result of this query is effective at leaving 0 and nulls out of the NTILEs function but the rest of the data is placed entirely in just 1 quintil like this:
+------------+---------+--+
| Atenciones | Quintil | |
+------------+---------+--+
| 0 | 0 | |
| 0 | 0 | |
| NULL | NULL | |
| 1 | 1 | |
| 3 | 1 | |
| 4 | 1 | |
| 7 | 1 | |
| 46 | 1 | |
| 59 | 1 | |
+------------+---------+--+
And I'm looking for something like this:
+------------+---------+--+
| Atenciones | Quintil | |
+------------+---------+--+
| 0 | 0 | |
| 0 | 0 | |
| NULL | NULL | |
| 1 | 1 | |
| 3 | 2 | |
| 4 | 2 | |
| 7 | 3 | |
| 46 | 4 | |
| 59 | 5 | |
+------------+---------+--+

Answer 1

Why define your own function? You can express this using case logic:
select (case when Atenciones is null or Atenciones = 0
then Atenciones
else ntile(5) over (partition by (case when Atenciones is null or Atenciones = 0 then 1 else 2 end)
order by Atenciones
)
end)