SQL Select only missing months - sql

Question

Notice the 2017-04-01, 2018-02-01, 2018-07-01, and 2019-01-01 months are missing in the output. I want to show only those months which are missing. Does anyone know how to go about this?
Query:
SELECT TO_DATE("Month", 'mon''yy') as dates FROM sample_sheet
group by dates
order by dates asc;
Output:
2017-01-01
2017-02-01
2017-03-01
2017-05-01
2017-06-01
2017-07-01
2017-08-01
2017-09-01
2017-10-01
2017-11-01
2017-12-01
2018-01-01
2018-03-01
2018-04-01
2018-05-01
2018-06-01
2018-08-01
2018-09-01
2018-10-01
2018-11-01
2018-12-01
2019-02-01
2019-03-01
2019-04-01

Answer 1

I don't know Vertica, so I wrote a working proof of concept in Microsoft SQL Server and tried to convert it to Vertica syntax based on the online documentation.
It should look like this:
with
months as (
select 2017 as date_year, 1 as date_month, to_date('2017-01-01', 'YYYY-MM-DD') as first_date, to_date('2017-01-31', 'yyyy-mm-dd') as last_date
union all
select
year(add_months(first_date, 1)) as date_year,
month(add_months(first_date, 1)) as date_month,
add_months(first_date, 1) as first_date,
last_day(add_months(first_date, 1)) as last_date
from months
where first_date < current_date
),
sample_dates (a_date) as (
select to_date('2017-01-15', 'YYYY-MM-DD') union all
select to_date('2017-01-22', 'YYYY-MM-DD') union all
select to_date('2017-02-01', 'YYYY-MM-DD') union all
select to_date('2017-04-15', 'YYYY-MM-DD') union all
select to_date('2017-06-15', 'YYYY-MM-DD')
)
select *
from sample_dates right join months on sample_dates.a_date between first_date and last_date
where sample_dates.a_date is null
Months is a recursive dynamic table that holds all months since 2017-01, with first and last day of the month. sample_dates is just a list of dates to test the logic - you should replace it with your own table.
Once you build that monthly calendar table all you need to do is check your dates against it using an outer query to see what dates are not between any of those periods between first_date and last_date columns.

Answer 2

You can build a TIMESERIES of all dates between the first input date and the last input date (The highest granularity of a TIMESERIES is the day.), and filter out only the months' first days out of that; then left join that created sequence of firsts of month with your input to find out where the join would fail, checking for NULLS from the input branch of the join:
WITH
-- your input
input(mth1st) AS (
SELECT DATE '2017-01-01'
UNION ALL SELECT DATE '2017-02-01'
UNION ALL SELECT DATE '2017-03-01'
UNION ALL SELECT DATE '2017-05-01'
UNION ALL SELECT DATE '2017-06-01'
UNION ALL SELECT DATE '2017-07-01'
UNION ALL SELECT DATE '2017-08-01'
UNION ALL SELECT DATE '2017-09-01'
UNION ALL SELECT DATE '2017-10-01'
UNION ALL SELECT DATE '2017-11-01'
UNION ALL SELECT DATE '2017-12-01'
UNION ALL SELECT DATE '2018-01-01'
UNION ALL SELECT DATE '2018-03-01'
UNION ALL SELECT DATE '2018-04-01'
UNION ALL SELECT DATE '2018-05-01'
UNION ALL SELECT DATE '2018-06-01'
UNION ALL SELECT DATE '2018-08-01'
UNION ALL SELECT DATE '2018-09-01'
UNION ALL SELECT DATE '2018-10-01'
UNION ALL SELECT DATE '2018-11-01'
UNION ALL SELECT DATE '2018-12-01'
UNION ALL SELECT DATE '2019-02-01'
UNION ALL SELECT DATE '2019-03-01'
UNION ALL SELECT DATE '2019-04-01'
)
,
-- need a series of month's firsts
-- TIMESERIES works for INTERVAL DAY TO SECOND
-- so build that timeseries, and filter out
-- the month's firsts
limits(mth1st) AS (
SELECT MIN(mth1st) FROM input
UNION ALL SELECT MAX(mth1st) FROM input
)
,
alldates AS (
SELECT dt::DATE FROM limits
TIMESERIES dt AS '1 day' OVER(ORDER BY mth1st::TIMESTAMP)
)
,
allfirsts(mth1st) AS (
SELECT dt FROM alldates WHERE DAY(dt)=1
)
SELECT
allfirsts.mth1st
FROM allfirsts
LEFT JOIN input USING(mth1st)
WHERE input.mth1st IS NULL;
-- out mth1st
-- out ------------
-- out 2017-04-01
-- out 2018-02-01
-- out 2018-07-01
-- out 2019-01-01