Get Average value for each X rows in SQL - mysql

Question

Let`s say I have the following table
+----+-------+
| Id | Value |
+----+-------+
| 1 | 2.0 |
| 2 | 8.0 |
| 3 | 3.0 |
| 4 | 9.0 |
| 5 | 1.0 |
| 6 | 4.0 |
| 7 | 2.5 |
| 8 | 6.5 |
+----+-------+
I want to plot these values, but since my real table has thousands of values, I thought about getting and average for each X rows. Is there any way for me to do so for, ie, each 2 or 4 rows, like below:
2
+-----+------+
| 1-2 | 5.0 |
| 3-4 | 6.0 |
| 5-6 | 2.5 |
| 7-8 | 4.5 |
+-----+------+
4
+-----+------+
| 1-4 | 5.5 |
| 5-8 | 3.5 |
+-----+------+
Also, is there any way to make this X value dynamic, based on the total number of rows in my table? Something like, if I have 1000 rows, the average will be calculated based on each 200 rows (1000/5), but if I have 20, calculate it based on each 4 rows (20/5).
I know how to do that programmatically, but is there any way to do so using pure SQL?
EDIT: I need it to work on mysql.

Answer 1

Depending on your DBMS, something like this will work:
SELECT
ChunkStart = Min(Id),
ChunkEnd = Max(Id),
Value = Avg(Value)
FROM
(
SELECT
Chunk = NTILE(5) OVER (ORDER BY Id),
*
FROM
YourTable
) AS T
GROUP BY
Chunk
ORDER BY
ChunkStart;
This creates 5 groups or chunks no matter how many rows there are, as you requested.
If you have no windowing functions you can fake it:
SELECT
ChunkStart = Min(Id),
ChunkEnd = Max(Id),
Value = Avg(Value)
FROM
YourTable
GROUP BY
(Id - 1) / (((SELECT Count(*) FROM YourTable) + 4) / 5)
;
I made some assumptions here such as Id beginning with 1 and there being no gaps, and that you would want the last group too small instead of too big if things didn't divide evenly. I also assumed integer division would result as in Ms SQL Server.

Answer 2

You can use modulos operator to act on every Nth row of the table. This example would get the average value for every 10th row:
select avg(Value) from some_table where id % 10 = 0;
You could then do a count of the rows in the table, apply some factor to that, and use that value as a dynamic interval:
select avg(Value) from some_table where id % (select round(count(*)/1000) from some_table) = 0;
You'll need to figure out the best interval based on the actual number of rows you have in the table of course.
EDIT:
Rereading you post I realize this is getting an average of every Nth row, and not each sequential N rows. I'm not sure if this would suffice, or if you specifically need sequential averages.

Answer 3

Look at the NTILE function (as in quartile, quintile, decile, percentile). You can use it to split your data evenly into a number of buckets - in your case it seems you would like five.
Then you can use AVG to calculate an average for each bucket.
NTILE is in SQL-99 so most DBMSes should have it.

Answer 4

You can try that
CREATE TABLE #YourTable
(
ID int
,[Value] float
)
INSERT #YourTable (ID, [Value]) VALUES
(1,2.0)
,(2,8.0)
,(3,3.0)
,(4,9.0)
,(5,1.0)
,(6,4.0)
,(7,2.5)
,(8,6.5)
SELECT
ID = MIN(ID) + '-' + MAX(ID)
,[Value] = AVG([Value])
FROM
(
SELECT
GRP = ((ROW_NUMBER() OVER(ORDER BY ID) -1) / 2) + 1
,ID = CONVERT(VARCHAR(10), ID)
,[Value]
FROM
#YourTable
) GrpTable
GROUP BY
GRP
DROP TABLE #YourTable