MySQL Average Every 4 Rows

I have a 10 million+ row table that looks like this:

    id | time_read  | value
    -----------------------
    9   1111111111   100
    9   1111111222   200
    9   1111111333   150
    9   1111111444   225

I want to AVG the value of every 4 rows. How do I do this in MySQL?

Also, secondary to my question, how might I do this if I wanted to avg the value of every 4 rows for all the data in a month and return that total for each month of each year in the table? For example, using the same table in my first question, ending up with:

    2008 | 12 | 500000
    2009 | 01 | 450000
    2009 | 02 | 475000

edit: In other words, the total of each average 4 rows grouped by year-month. Each time_read is 15 minutes apart.

I was doing something like this previously, but it wasn't accurate enough. I need to average every 4 rows and total that instead o开发者_开发技巧f taking the sum of all value's in a month and dividing by 4.

    SELECT DATE_FORMAT(FROM_UNIXTIME(time_read),'%Y %m') as tr,  
    DATE_FORMAT(FROM_UNIXTIME(time_read),'%Y') as year, 
    DATE_FORMAT(FROM_UNIXTIME(time_read),'%m') as month, SUM(value) as value 
    FROM table WHERE id = 9 
    GROUP BY tr

---

If you don't want to rely on your "id" column (it might not be sequential), you can just create your own variable to group by that changes every 4 rows, like so:

SET @rank = 0;
SELECT AVG(value), CEIL((@rank:=@rank+1)/4) AS rank FROM "your_table" GROUP BY rank;

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I don't know if you did this on purpose, but I think it's just a little typo: you're ID is 9 for every row.
If it is, you may be looking for this query:

SELECT AVG(value) AS val, CEIL(id / 4) AS group_id FROM table GROUP BY group_id

2nd question:

SELECT SUM(val) FROM (SELECT AVG(value) AS val, CEIL(id / 4) AS group_id FROM table GROUP BY group_id) AS fred

At this time I cannot provide a where-part for the second question, since I don't know how you're filtering things by date (I don't see a date-column or something). So for now, this will calculate the sum of all average values grouped by 4.

---

Try this code -

CREATE TABLE table1 (
  id INT(11) NOT NULL AUTO_INCREMENT,
  time_read INT(11) DEFAULT NULL,
  value INT(11) DEFAULT NULL,
  PRIMARY KEY (id)
);

INSERT INTO table1 VALUES 
  (1, 1312880400, 10),  -- 09.08.2011 12:00:00 -> 1 august
  (2, 1312880410, 20),  -- 09.08.2011 12:00:10 -> 2
  (3, 1312880420, 30),  -- 09.08.2011 12:00:20 -> 3
  (4, 1312880430, 40),  -- 09.08.2011 12:00:30 -> 4
  (5, 1312880440, 50),  -- 09.08.2011 12:00:40 -> 5
  (6, 1315558800, 60),  -- 09.09.2011 12:00:00 -> 1 september
  (7, 1315558810, 70);  -- 09.09.2011 12:00:10 -> 2

SELECT AVG(value) FROM (
  SELECT t1.*, COUNT(*) cnt FROM table1 t1
  LEFT JOIN table1 t2
    ON t2.time_read <= t1.time_read
      AND YEAR(FROM_UNIXTIME(t2.time_read)) = YEAR(FROM_UNIXTIME(t1.time_read))
      AND MONTH(FROM_UNIXTIME(t2.time_read)) = MONTH(FROM_UNIXTIME(t1.time_read))
  GROUP 
    BY time_read
) t
GROUP BY
  YEAR(FROM_UNIXTIME(time_read)), MONTH(FROM_UNIXTIME(time_read)), CEIL(cnt/4);

+------------+
| AVG(value) |
+------------+
|    25.0000 |
|    50.0000 |
|    65.0000 |
+------------+

It does group by month and 4 records in month.