SQL query to group by day

Question

I want to list all sales, and group the sum by day.

Sales (saleID INT, amount INT, created DATETIME)

NOTE: I am using SQL Server 2005.

Answer 1

For SQL Server:

GROUP BY datepart(year,datefield), 
    datepart(month,datefield), 
    datepart(day,datefield)

or faster (from Q8-Coder):

GROUP BY dateadd(DAY,0, datediff(day,0, created))

For MySQL:

GROUP BY year(datefield), month(datefield), day(datefield)

or better (from Jon Bright):

GROUP BY date(datefield)

For Oracle:

GROUP BY to_char(datefield, 'yyyy-mm-dd')

or faster (from IronGoofy):

GROUP BY trunc(created);

For Informix (by Jonathan Leffler):

GROUP BY date_column
GROUP BY EXTEND(datetime_column, YEAR TO DAY)

Answer 2

If you're using SQL Server, you could add three calculated fields to your table:

Sales (saleID INT, amount INT, created DATETIME)

ALTER TABLE dbo.Sales
  ADD SaleYear AS YEAR(Created) PERSISTED
ALTER TABLE dbo.Sales
  ADD SaleMonth AS MONTH(Created) PERSISTED
ALTER TABLE dbo.Sales
  ADD SaleDay AS DAY(Created) PERSISTED

and now you could easily group by, order by etc. by day, month or year of the sale:

SELECT SaleDay, SUM(Amount)
FROM dbo.Sales
GROUP BY SaleDay

Those calculated fields will always be kept up to date (when your "Created" date changes), they're part of your table, they can be used just like regular fields, and can even be indexed (if they're "PERSISTED") - great feature that's totally underused, IMHO.

Marc