Getting missing time period value with an interval in My SQL - mysql

Question

I'm trying to fetch the records with half an hour time interval of the current day with concern data count for that time period.
So, my output came as expected. But, If count(no records) on the particular time period let's say 7:00 - 7:30 I'm not getting that record with zero count.
My Query as follows :
SELECT time_format( FROM_UNIXTIME(ROUND(UNIX_TIMESTAMP(start_time)/(30* 60)) * (30*60)) , '%H:%i')
thirtyHourInterval , COUNT(bot_id) AS Count FROM bot_activity
WHERE start_time BETWEEN CONCAT(CURDATE(), ' 00:00:00') AND CONCAT(CURDATE(), ' 23:59:59')
GROUP BY ROUND(UNIX_TIMESTAMP(start_time)/(30* 60))
For reference of my output :

Answer 1

We need a source for that 7:30 row; a row source for all the time values.
If we have a clock table that contains all of the time values we want to return, such that we can write a query that returns that first column, the thirty minute interval values we want to return,
as an example:
SELECT c.hhmm AS thirty_minute_interval
FROM clock c
WHERE c.hhmm ...
ORDER BY c.hhmm
then we can do an outer join the results of the query with the missing rows
SELECT c.hhmm AS _thirty_minute_interval
, IFNULL(r._cnt_bot,0) AS _cnt_bot
FROM clock c
LEFT
JOIN ( -- query with missing rows
SELECT time_format(...) AS thirtyMinuteInterval
, COUNT(...) AS _cnt_bot
FROM bot_activity
WHERE
GROUP BY time_format(...)
) r
ON r.thirtyMinuteInterval = c.hhmm
WHERE c.hhmm ...
ORDER BY c.hhmm
The point is that the SELECT will not generate "missing" rows from a source where they don't exist; we need a source for them. We don't necessarily have to have a separate clock table, we could have an inline view generate the rows. But we do need to be able to SELECT those value from a source.
( Note that bot_id in the original query is indeterminate; the value will be from some row in the collapsed set of rows, but no guarantee which value. (If we add ONLY_FULL_GROUP_BY to sql_mode, the query will throw an error, like most other relational databases will when non-aggregate expressions in the SELECT list don't appear in the GROUP BY are aren't functionally dependent on the GROUP BY )
EDIT
In place of a clock table, we can use an inline view. For small sets, we could something like this.
SELECT c.tmi
FROM ( -- thirty minute interval
SELECT CONVERT(0,TIME) + INTERVAL h.h+r.h HOUR + INTERVAL m.mm MINUTE AS tmi
FROM ( SELECT 0 AS h UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3
UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7
UNION ALL SELECT 8 UNION ALL SELECT 9 UNION ALL SELECT 10 UNION ALL SELECT 11
) h
CROSS JOIN ( SELECT 0 AS h UNION ALL SELECT 12 ) r
CROSS JOIN ( SELECT 0 AS mm UNION ALL SELECT 30 ) m
ORDER BY tmi
) c
ORDER
BY c.tmi
(Inline view c is a standin for a clock table, returns time values on thirty minute boundaries.)
That's kind of ugly. We can see where if we had a rowsource of just integer values, we could make this much simpler. But if we pick that apart, we can see how to extend the same pattern to generate fifteen minute intervals, or shorten it to generate two hour intervals.