Question

I want to use the AVG function in sql to return a working average for some values (ie based on the last week not an overall average). I have two values I am calculating, weight and restingHR (heart rate). I have the following sql statements for each:

SELECT AVG( weight ) AS average 
FROM   stats 
WHERE  userid='$userid' 
ORDER BY date DESC LIMIT 7

SELECT AVG( restingHR ) AS average 
FROM   stats 
WHERE  userid='$userid' 
ORDER BY date DESC LIMIT 7

The value I get for weight is 82.56 but it should be 83.35
This is not a massive error and I’m rounding it when I use it so its not too big a deal.

However for restingHR I get 45.96 when it should be 57.57 which is a massive difference.

I don’t understand why this is going so wrong. Any help is much appreciated.

Thanks

Answer 1

Use a subquery to separate selecting the rows from computing the average:

SELECT AVG(weight) average
FROM (SELECT weight
      FROM stats
      WHERE userid = '$userid'
      ORDER BY date DESC
      LIMIT 7) subq