SQL moving average

Question

How do you create a moving average in SQL?

Current table:

Date             Clicks 
2012-05-01       2,230
2012-05-02       3,150
2012-05-03       5,520
2012-05-04       1,330
2012-05-05       2,260
2012-05-06       3,540
2012-05-07       2,330

Desired table or output:

Date             Clicks    3 day Moving Average
2012-05-01       2,230
2012-05-02       3,150
2012-05-03       5,520          4,360
2012-05-04       1,330          3,330
2012-05-05       2,260          3,120
2012-05-06       3,540          3,320
2012-05-07       2,330          3,010

Answer 1

This is an Evergreen Joe Celko question. I ignore which DBMS platform is used. But in any case Joe was able to answer more than 10 years ago with standard SQL.

Joe Celko SQL Puzzles and Answers citation: "That last update attempt suggests that we could use the predicate to construct a query that would give us a moving average:"

SELECT S1.sample_time, AVG(S2.load) AS avg_prev_hour_load
FROM Samples AS S1, Samples AS S2
WHERE S2.sample_time
BETWEEN (S1.sample_time - INTERVAL 1 HOUR)
AND S1.sample_time
GROUP BY S1.sample_time;

Is the extra column or the query approach better? The query is technically better because the UPDATE approach will denormalize the database. However, if the historical data being recorded is not going to change and computing the moving average is expensive, you might consider using the column approach.

MS SQL Example:

CREATE TABLE #TestDW
( Date1 datetime,
  LoadValue Numeric(13,6)
);

INSERT INTO #TestDW VALUES('2012-06-09' , '3.540' );
INSERT INTO #TestDW VALUES('2012-06-08' , '2.260' );
INSERT INTO #TestDW VALUES('2012-06-07' , '1.330' );
INSERT INTO #TestDW VALUES('2012-06-06' , '5.520' );
INSERT INTO #TestDW VALUES('2012-06-05' , '3.150' );
INSERT INTO #TestDW VALUES('2012-06-04' , '2.230' );

SQL Puzzle query:

SELECT S1.date1,  AVG(S2.LoadValue) AS avg_prev_3_days
FROM #TestDW AS S1, #TestDW AS S2
WHERE S2.date1
    BETWEEN DATEADD(d, -2, S1.date1 )
    AND S1.date1
GROUP BY S1.date1
order by 1;

Answer 2

One way to do this is to join on the same table a few times.

select
 (Current.Clicks 
  + isnull(P1.Clicks, 0)
  + isnull(P2.Clicks, 0)
  + isnull(P3.Clicks, 0)) / 4 as MovingAvg3
from
 MyTable as Current
 left join MyTable as P1 on P1.Date = DateAdd(day, -1, Current.Date)
 left join MyTable as P2 on P2.Date = DateAdd(day, -2, Current.Date)
 left join MyTable as P3 on P3.Date = DateAdd(day, -3, Current.Date)

Adjust the DateAdd component of the ON-Clauses to match whether you want your moving average to be strictly from the past-through-now or days-ago through days-ahead.

• This works nicely for situations where you need a moving average over only a few data points.
• This is not an optimal solution for moving averages with more than a few data points.