Calculate Average for the month in SQlite - sqlite

Question

I have an Sqlite table as below called balance history:
Table Balance History
Date Amount
2013-11-01 16:26:52 1000
2013-11-15 13:20:52 2000
2013-11-27 12:26:55 3000
I would like to calculate the average for the month.
**
The Expected OutPut will be 1666.67
**
Which will be (1000 * 14 days + 2000 * 12 days + 3000 * 4 days)/30 days
= (14000 + 24000 + 12000)/30 = 1666.67
How can I achieve this in SQlite? any help will be appreciated.
thanks

Answer 1

First, we have to compute the start and the end of each interval.
The simple query (SELECT Date AS "From", Amount FROM BalanceHistory WHERE Date GLOB '2013-11*') gets the start for each interval in the month.
(GLOB is case sensitive and thus allows to use a normal index; LIKE would require a special case-insensitive index.)
If there is no record for the first day of the month, the part after the UNION ALL adds the last record of the previous month and changes the day to the 1st.
The COALESCE computes the end of the interval.
The subquery gets the next date from the table, if there is one in the current month.
If there is no such record, it takes the first day of the next month:
SELECT date("From") AS "From",
COALESCE((SELECT date(MIN(Date))
FROM BalanceHistory
WHERE Date > "From"
AND Date GLOB '2013-11*'),
date('2013-11-01', '+1 month')
) AS "To",
"Amount"
FROM (SELECT Date AS "From",
Amount
FROM BalanceHistory
WHERE Date GLOB '2013-11*'
UNION ALL
SELECT *
FROM (SELECT date(Date, '+1 month', 'start of month'),
Amount
FROM BalanceHistory
WHERE Date < '2013-11'
AND NOT EXISTS (SELECT 1
FROM BalanceHistory
WHERE Date GLOB '2013-11-01*')
ORDER BY Date DESC
LIMIT 1)
);
From To Amount
---------- ---------- ------
2013-11-01 2013-11-15 1000
2013-11-15 2013-11-27 2000
2013-11-27 2013-12-01 3000
We can then wrap this in another query to compute the number of days, and add them up.
The strftime calculates the last day of the month, i.e., the number of days:
SELECT SUM((julianday("To") - julianday("From")) * Amount) /
strftime('%d', '2013-11-01', '+1 month', '-1 day') AS MonthAvg
FROM (SELECT date("From") AS "From",
COALESCE((SELECT date(MIN(Date))
FROM BalanceHistory
WHERE Date > "From"
AND Date GLOB '2013-11*'),
date('2013-11-01', '+1 month')
) AS "To",
"Amount"
FROM (SELECT Date AS "From",
Amount
FROM BalanceHistory
WHERE Date GLOB '2013-11*'
UNION ALL
SELECT *
FROM (SELECT date(Date, '+1 month', 'start of month'),
Amount
FROM BalanceHistory
WHERE Date < '2013-11'
AND NOT EXISTS (SELECT 1
FROM BalanceHistory
WHERE Date GLOB '2013-11-01*')
ORDER BY Date DESC
LIMIT 1)
)
)
And while we're at it, we can wrap this in yet another query to replace all those 2013-11 strings with the month as read from the table.
This allows as to compute this for every month:
SELECT Month,
(SELECT SUM((julianday("To") - julianday("From")) * Amount) /
strftime('%d', Month || '-01', '+1 month', '-1 day')
FROM (SELECT date("From") AS "From",
COALESCE((SELECT date(MIN(Date))
FROM BalanceHistory
WHERE Date > "From"
AND Date GLOB Month || '*'),
date(Month || '-01', '+1 month')
) AS "To",
"Amount"
FROM (SELECT Date AS "From",
Amount
FROM BalanceHistory
WHERE Date GLOB Month || '*'
UNION ALL
SELECT *
FROM (SELECT date(Date, '+1 month', 'start of month'),
Amount
FROM BalanceHistory
WHERE Date < Month
AND NOT EXISTS (SELECT 1
FROM BalanceHistory
WHERE Date GLOB Month || '-01*')
ORDER BY Date DESC
LIMIT 1)
)
)
) AS MonthAvg
FROM (SELECT DISTINCT strftime('%Y-%m', Date) AS Month
FROM BalanceHistory)
ORDER BY 1