[sql] MySQL/SQL: Group by date only on a Datetime column

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.

Anyone know how to do this? I can still do it (as I am atm), dynamically in my code, but I'm cleaning trash code and this can be made through the SQL I just can't find out how :(.

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.

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 

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.

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!

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 

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!

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.