Question

Is there an equivalent to 1/countif in SQL Server?

Something like:

SELECT ID,1/COUNTIF(ID)
FROM Table

…that divides 1 by the number of rows with a certain ID in Table, returning the results in this format:

ID  Result
1   0.5
1   0.5 
2   1
3   0.33
3   0.33
3   0.33 etc.

Answer 1

If your version is 2005 and newer, you can use OVER with the aggregate COUNT():

SELECT id
     , 1.0 / COUNT(*) OVER (PARTITION BY id) AS countif_ID
FROM TableX ;

You can find more examples in the MSDN site: OVER clause.