Group by date only on a Datetime column

Question

Having a table with a column like: mydate DATETIME ...

I have a query such as:

SELECT SUM(foo), mydate FROM a_table GROUP BY a_table.mydate;

This will group by the full datetime, including hours and minutes. I wish to make the group by, only by the date YYYY/MM/DD not by the YYYY/MM/DD/HH/mm.

How to do this?

Answer 1

Cast the datetime to a date, then GROUP BY using this syntax:

SELECT SUM(foo), DATE(mydate) FROM a_table GROUP BY DATE(a_table.mydate);

Or you can GROUP BY the alias as @orlandu63 suggested:

SELECT SUM(foo), DATE(mydate) DateOnly FROM a_table GROUP BY DateOnly;

Though I don't think it'll make any difference to performance, it is a little clearer.

Answer 2

I found that I needed to group by the month and year so neither of the above worked for me. Instead I used date_format

SELECT date
FROM blog 
GROUP BY DATE_FORMAT(date, "%m-%y")
ORDER BY YEAR(date) DESC, MONTH(date) DESC 

Answer 3

Or:

SELECT SUM(foo), DATE(mydate) mydate FROM a_table GROUP BY mydate;

More efficient (I think.) Because you don't have to cast mydate twice per row.

Answer 4

SELECT SUM(No), HOUR(dateofissue) 
FROM tablename 
WHERE dateofissue>='2011-07-30' 
GROUP BY HOUR(dateofissue)

It will give the hour by sum from a particular day!