Unrolling periodical events in MySQL - mysql

Question

I've got a database with two tables, that I want to combine. One of the tables contains "incidental events", which just occur once. Next to this, I also have "periodical events". Now I want to combine these two in a view.
The incidental one simply has two columns, one called changes, the other one called date. The periodical one has three columns, changes, startDate and endDate. The difference between these two can be a maximum of 50 years, so manually typing out one case for every day is not going to work. Both views also have an AI ID. In this view I want to have a column date and a column changes.
To achieve this I want to unroll the periodical changes table, so that it shows one entry for every day in between the startDate and endDate. For instance:
incidental changes:
date | change
09/08/2015 | 5
11/08/2015 | 10
periodical changes:
startDate | endDate | change
09/08/2015 | 12/08/2015 | 7
These two I want combined into:
changes view:
date | change
09/08/2015 | 5
09/08/2015 | 7
10/08/2015 | 7
11/08/2015 | 10
11/08/2015 | 7
12/08/2015 | 7
My idea is to use something like this:
SELECT * FROM incidental_changes,(
SET #id = (SELECT min(ID) AS min FROM periodical_changes WHERE 1)
SET #maxID = (SELECT max(ID) AS max FROM periodical_changes WHERE 1)
WHILE (#id <= #maxID) DO
SET #firstDate = (SELECT startDate FROM periodical_changes WHERE id = #id)
SET #lastDate = (SELECT endDate FROM periodical_changes WHERE id = #id)
WHILE (#firstDate <= #lastDate) DO
SELECT #firstDate AS date, change FROM periodical_changes WHERE id = #id
#firstDate = #firstDate + INTERVAL 1 DAY
END
#id = #id + 1
END
) WHERE 1
This gives me an error,
CREATE ALGORITHM = UNDEFINED VIEW all_periodicals AS SELECT * FROM
incidental_changes,( SET #id = (SELECT min(ID) AS min FROM
periodical_changes WHERE 1) SET #maxID = (SELECT max(ID) AS max FROM
periodical_changes WHERE 1) WHILE (#id <= #maxID) DO SET #firstDate =
(SELECT startDate FROM periodical_changes WHERE id = #id) SET
#lastDate = (SELECT endDate FROM periodical_changes WHERE id = #id)
WHILE (#firstDate <= #lastDate) DO SELECT #firstDate AS date, change
FROM periodical_changes WHERE id = #id #firstDate = #firstDate +
INTERVAL 1 DAY END #id = #id + 1 END ) WHERE 1
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use
near 'SET #id = (SELECT min(ID) AS min FROM periodical_changes WHERE
1) SET #' at line 5
and I'm guessing that if I'd manage to fix this error there'd be more. So, is there any way to do this the way I want, or do I have to look for a different approach?
EDIT:
Okay, so far I have not found a way to do this in a view or so. So instead I am now using a routine. This routine has one parameter, account INT. The definition I am using so far is as followed:
BEGIN
DECLARE periodicalID int;
DECLARE v_finished INTEGER DEFAULT 0;
DECLARE periodicalCursor CURSOR
FOR SELECT periodicals.periodicalID FROM periodicals WHERE periodicals.accountID = account;
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET v_finished = 1;
CREATE TEMPORARY TABLE results LIKE incidentials;
ALTER TABLE results DROP INDEX date;
SET #periodicalID = -1;
OPEN periodicalCursor;
allPeriodicals: LOOP
FETCH periodicalCursor INTO periodicalID;
IF (v_finished) THEN
LEAVE allPeriodicals;
END IF;
SELECT periodicals.startDate,periodicals.numberOfPeriods,periodicals.period,periodicals.endDate,periodicals.money FROM periodicals WHERE periodicals.periodicalID = periodicalID AND periodicals.accountID = account INTO #startDate, #numberOfPeriods, #period,#endDate,#money;
SET #intervalStatement = "SELECT ? + INTERVAL ? ";
SET #intervalStatement = CONCAT(#intervalStatement,#period," INTO #res");
PREPARE intervalStatement FROM #intervalStatement;
WHILE #startDate <= #endDate DO
EXECUTE intervalStatement USING #startDate,#numberOfPeriods;
SET #startDate = #res;
INSERT INTO results(accountID,date,money) VALUES (account,#startDate,#money);
END WHILE;
END LOOP allPeriodicals;
INSERT INTO results(accountID,date,money) SELECT accountID,date, money FROM incidentials WHERE incidentials.accountID = account;
SELECT * FROM results ORDER BY date;
END
This poses the problem of performance though. With only one periodical entry spread over a year this query already takes about 16 seconds. So even though this approach works, I either did something wrong causing it to take this long or this is not the right way to go.

Answer 1

Let me presume you have a numbers table. Then you can do:
select i.date, i.change
from incidental
union all
select date_add(p.startDate, interval n.n - 1 day), p.change
from periodic p join
numbers n
on date_add(p.startDate, interval n.n - 1 day) <= p.endDate;
For a select query, you can generate the numbers using a subquery, if you know the maximum length. Something like:
select i.date, i.change
from incidental
union all
select date_add(p.startDate, interval n.n - 1 day), p.change
from periodic p join
(select 1 as n union all select 2 union all select 3 union all select 4 union all
select 5 union all select 6 union all select 7
) n
on date_add(p.startDate, interval n.n - 1 day) <= p.endDate;
This doesn't work in a view, however. For that, you really do need a numbers table.