Calculate average from data only stored in MySQL on change - mysql

Question

We have a database where we only store data values and timestamp (as a row in MySQL) when the data value changes. There are therefore no fixed interval between the timestamps in the database. The table looks something like this:
MySQLTimestamp Data
2014-10-01 18:01 1
2014-10-03 16:13 2
2014-10-07 15:45 1
2014-10-09 10:08 3
THE PROBLEM: We want to calculate the average over time assuming that the data value continues to be i.e. 2 until the value changes on the next row in the database.
A simple AVG won't do the trick because it will only calculate the average between the number of rows. This would not take in to count that a value can continue to be the same value for a long period of time before the next row states a change in data value.
Would really appreciate your help!

Answer 1

Self join and calculate the duration of date or time as weight to Data.
select
sum(data*duration_of_date)/sum(duration_of_date) as avg_over_date,
sum(data*duration_of_hour)/sum(duration_of_hour) as avg_over_hour,
sum(data*duration_of_sec)/sum(duration_of_sec) as avg_over_sec
from (
select
t1.MySQLTimestamp,
t1.data,
min(case when t1.MySQLTimestamp<t2.MySQLTimestamp
then t2.MySQLTimestamp else null end) as next_tm,
datediff(
min(case when t1.MySQLTimestamp<t2.MySQLTimestamp
then t2.MySQLTimestamp else null end) ,
t1.MySQLTimestamp) as duration_of_date,
TIME_TO_SEC(timediff(
min(case when t1.MySQLTimestamp<t2.MySQLTimestamp
then t2.MySQLTimestamp else null end) ,
t1.MySQLTimestamp))/60/60 as duration_of_hour,
TIME_TO_SEC(timediff(
min(case when t1.MySQLTimestamp<t2.MySQLTimestamp
then t2.MySQLTimestamp else null end) ,
t1.MySQLTimestamp)) as duration_of_sec
from
your_table t1
cross join
your_table t2
group by
t1.MySQLTimestamp,
t1.data
) as t
Use datediff to calculate days interval as weight. If you want hours or minutes as interval, you could use timediff and transfer the result to hours, minutes or seconds.
Here is the sql fiddle demo and the results:
AVG_OVER_DATE AVG_OVER_HOUR AVG_OVER_SEC
1.5 1.51887 1.5189
Another version by left join:
select
sum(data*duration_of_date)/sum(duration_of_date) as avg_over_date,
sum(data*duration_of_hour)/sum(duration_of_hour) as avg_over_hour,
sum(data*duration_of_sec)/sum(duration_of_sec) as avg_over_sec
from (
select
t1.MySQLTimestamp,
t1.data,
min(t2.MySQLTimestamp) as next_tm,
datediff(min(t2.MySQLTimestamp), t1.MySQLTimestamp) as duration_of_date,
TIME_TO_SEC(timediff(min(t2.MySQLTimestamp), t1.MySQLTimestamp))/60/60 as duration_of_hour,
TIME_TO_SEC(timediff(min(t2.MySQLTimestamp), t1.MySQLTimestamp)) as duration_of_sec
from
your_table t1
left join
your_table t2
on
t1.MySQLTimestamp<t2.MySQLTimestamp
group by
t1.MySQLTimestamp,
t1.data
) as t