How to quickly select DISTINCT dates from a Date/Time field, SQL Server

Question

I am wondering if there is a good-performing query to select distinct dates (ignoring times) from a table with a datetime field in SQL Server.

My problem isn\'t getting

Answer 1

Update:

Solution below tested for efficiency on a 2M table and takes but 40 ms.

Plain DISTINCT on an indexed computed column took 9 seconds.

See this entry in my blog for performance details:

• SQL Server: efficient DISTINCT on dates

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Unfortunately, SQL Server's optimizer can do neither Oracle's SKIP SCAN nor MySQL's INDEX FOR GROUP-BY.

It's always Stream Aggregate that takes long.

You can built a list of possible dates using a recursive CTE and join it with your table:

WITH    rows AS (
        SELECT  CAST(CAST(CAST(MIN(date) AS FLOAT) AS INTEGER) AS DATETIME) AS mindate, MAX(date) AS maxdate
        FROM    mytable
        UNION ALL
        SELECT  mindate + 1, maxdate
        FROM    rows
        WHERE   mindate < maxdate
        )
SELECT  mindate
FROM    rows
WHERE   EXISTS
        (
        SELECT  NULL
        FROM    mytable
        WHERE   date >= mindate
                AND date < mindate + 1
        )
OPTION  (MAXRECURSION 0)

This will be more efficient than Stream Aggregate