Using DISTINCT in window function with OVER

I'm trying to migrate a query from Oracle to SQL Server 2014.

Here is my query which works great in Oracle:

select
count(distinct A) over (partition by B) / count(*) over() as A_B
from MyTable

Here is the error i got after tried to run this query in SQL Server 2014.

Use of DISTINCT is not allowed with the OVER clause

Anyone know what is the problem? Is such as kind of query possible in SQL Server?
Please advise.

Anyone know what is the problem? Is such as kind of query possible in
SQL Server?

No it isn't currently implemented. See the following connect item request.

OVER clause enhancement request - DISTINCT clause for aggregate functions

Another possible variant would be

SELECT M.A,
       M.B,
       T.A_B
FROM   MyTable M
       JOIN (SELECT CAST(COUNT(DISTINCT A) AS NUMERIC(18,8)) / SUM(COUNT(*)) OVER() AS A_B,
                    B
             FROM   MyTable
             GROUP  BY B) T
         ON EXISTS (SELECT M.B INTERSECT SELECT T.B)

the cast to NUMERIC is there to avoid integer division. The reason for the join clause is explained here.

It can be replaced with ON M.B = T.B OR (M.B IS NULL AND T.B IS NULL) if preferred (or simply ON M.B = T.B if the B column is not nullable).

StackExchange Database Administrators Q#89031, answer score: 14