SQL Count Totals Within Date Ranges

I have a CHANGES table with fields VALUE(integer) and CREATED_AT(timestamp). I want to know the total of the VALUE column grouped by each of the past 30 days (without making 30 queries).

So if yeste开发者_开发知识库rday there were records created with VALUEs of 10, -7, and 12; I would want a record returned with CREATED_AT = yesterday and TOTAL = 15.

Any help?

SELECT date(created_at) as CREATED_AT, sum(value) as TOTAL
FROM changes
WHERE created_at >= curdate() - interval 30 day
GROUP BY date(created_at);

Well, it slightly depends on what kind the timestamp is formatted in (SQL/ Unix/ etc). But this type of query might help you along:

SELECT
   DATE_FORMAT(CREATED_AT, '%Y-%m-%d') ym,
   COUNT(VALUE)
FROM foo
GROUP BY ym