Group by datetime ignoring time portion

Question

Is it possible to write a Microsoft SQL query that will group by a datetime data-type but ignoring the time part such as the hour and minute?

Answer 1

If you have a large table and want grouping to happen fast, you cannot rely grouping on an expression.

Create an additional datetime column on your table and have it filled by a trigger that calculates the base date from you "real" date:

UPDATE
  MyTable
SET
  EffectiveDate = DATEADD(DAY, 0, DATEDIFF(DAY, 0, t.RecordDate))
FROM
  MyTable t
  INNER JOIN inserted i ON i.RowID = t.RowID

Then put an index on EffectiveDate and grouping (and searching) will be a lot faster.