Date and time values are not entirely intuitive to aggregate into averages in T-SQL, although the business case does arguably exist. Suppose, for instance, that you have a production log with a “duration” column (in the “time” datatype), and you want to find the totalt or average duration for a certain group of items.
It’s possible, but I would still call it a workaround.
Continue readingYou may have discovered that the use of DISTINCT is not supported in windowed functions. A query that uses a distinct aggregate in a windowed function,
SELECT COUNT(DISTINCT something) OVER (PARTITION BY other) FROM somewhere;
will generate the following error message:
Msg 10759, Level 15, State 1, Line 1 Use of DISTINCT is not allowed with the OVER clause.
There are, however, a few relatively simple workarounds that are suprisingly efficient.
This article will show you how to create a “top 10” list of data, as well as providing an aggregate on the remainder of the data. There are a few different approaches to this.
Windowed functions are a powerful feature of T-SQL, allowing you to perform advanced aggregates. They provide a very efficient way of doing this as soon as you just get the hang of the OVER() clause.
sqlsunday.com 