Hourly gaps in data for a month - sql

Question

Hi Fellow StackOverflowers
I'm really struggling with finding the right approach for this (in my mind) relatively simple Gap-finding problem.
I have a table with hourly datetimes (hourly log file imported to DB).
I need to find the missing hours during a period (lets say april).
So imagine having the following data in DB table [imported_logs]
[2018-04-02 10:00:000]
[2018-04-02 11:00:000]
[2018-04-02 12:00:000]
[2018-04-02 17:00:000]
i need the result for april gap analysis to be:
[ GAP-BEGIN ] [ GAB_END ]
[2018-04-01 00:00:000] [2018-04-02 10:00:000] <-- problem
[2018-04-02 13:00:000] [2018-04-02 17:00:000] <-- can be found using below code
[2018-04-02 18:00:000] [2018-05-01 00:00:000] <-- problem
My problem is mainly finding the start and end ranges, but the following code is helping finding the cap in between available data.
WITH t AS (
SELECT *, rn = ROW_NUMBER() OVER (PARTITION BY zone ORDER BY hourImported)
FROM logsImportedTable
Where hourImported > '2018-04-01' and hourImported < '2018-05-01' and zone = 1
)
SELECT t1.zone, t1.hourImported as GapStart, t2.hourImported as GapEnd
FROM t t1
INNER JOIN t t2 ON t2.zone = t1.zone AND t2.rn = t1.rn + 1
WHERE DATEDIFF(MINUTE, t1.hourImported, t2.hourImported) > 60
which gives me only the result:
[zone] [gap_start ] [gap_end ]
[1 ] [2018-04-02 13:00:00.000] [2018-04-02 17:00:00.000]
So basically if no logs has been imported during april at all the current implemetation would show no missing data at all (kinda wrong)
I'm thinking that i need to somehow add some new datapoints just before beginning and end of the april period to somehow get the query to catch the start and end of the month as missing data?
What would you bright guys/girls do?
/Kind regards

Answer 1

For this situation, just add the initial and end values, if appropriate:
<your query here>
union all
select '2018-04-01 00:00:000', min(lit.hourImported)
from logsImportedTable lit
where lit.hourImported >= '2018-04-01 00:00:00'
having min(lit.hourImported) > '2018-04-01 00:00:00'
union all
select '2018-05-01 00:00:000', max(lit.hourImported)
from logsImportedTable lit
where lit.hourImported <= '2018-05-01 00:00:00'
having max(lit.hourImported) > '2018-05-01 00:00:00';

Answer 2

Ok thx to the great help from #Gordon this is my final solution to the problem. It will give the gaps even if entire month of data is missing and all small gaps within.
DECLARE #zone INT = 1, #currentPeriodStart DATETIME = '2018-01-01',
#currentPeriodEnd DATETIME = '2018-02-01';
WITH t AS (
SELECT *, rn = ROW_NUMBER() over (PARTITION BY zone_id ORDER BY
time_of_file_present)
FROM test
Where time_of_file_present > #currentPeriodStart and time_of_file_present <
#currentPeriodEnd and zone_id = #zone
)
SELECT t1.zone_id, t1.time_of_file_present as gap_start,
t2.time_of_file_present as gap_end
FROM t t1
INNER JOIN t t2 ON t2.zone_id = t1.zone_id AND t2.rn = t1.rn + 1
WHERE DATEDIFF(MINUTE, t1.time_of_file_present, t2.time_of_file_present) >60
union all
select #zone, #currentPeriodStart, min(lit.time_of_file_present)
from test lit
where lit.time_of_file_present >= #currentPeriodStart
having min(lit.time_of_file_present) > #currentPeriodStart and
min(lit.time_of_file_present) < #currentPeriodEnd
union all
select #zone,max(lit.time_of_file_present), #currentPeriodEnd
from test lit
where lit.time_of_file_present <= #currentPeriodEnd
having max(lit.time_of_file_present) < #currentPeriodEnd and
max(lit.time_of_file_present) > #currentPeriodStart
union all
select #zone,#currentPeriodStart, #currentPeriodEnd
from test lit
having max(lit.time_of_file_present) < #currentPeriodStart or
max(lit.time_of_file_present) > #currentPeriodEnd
order by gap_start