Aggregating date and time values

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.

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Windowed DISTINCT aggregates

You 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,

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.

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