SQL Query to find streak with unique record types - sql

Question

This is my example table data:
CustomerType Date Records
1 2018-01-01 233
2 2018-01-01 12
1 2018-01-02 0
2 2018-01-02 34
1 2018-01-03 0
2 2018-01-03 35
1 2018-01-04 0
2 2018-01-04 0
1 2018-01-05 5562
2 2018-01-05 3
I would like the output to show the maximum 0 Record streak, by CustomerType, to be fetched for each CustomerType. In other words, I would like the maximum number of zero records in a row for each particular CustomerType
Example output
CustomerType MaxZeroRecordsStreak
1 3
2 1
The farthest I've gotten so far is something like this:
SELECT CustomerType,
(SELECT COUNT(*)
FROM Table1 as t1
AND Records = '0'
AND t1.Date <= t2.Date) AS MaxZeroRecordsStreak
FROM Table2 As t2
ORDER BY CustomerType ASC;
But this is clearly not what I want.
Any help with this complex query would be appreciated.

Answer 1

Most databases support the ANSI-standard window functions. You can do this using the difference of row_numbers(). The following gets all sequences of zeros for a customer type:
select customertype, count(*) as num_zeros
from (select t.*,
row_number() over (partition by records, customertype order by date) as seqnum_rt,
row_number() over (partition by customertype order by date) as seqnum_t
from t
) t
where records = 0
group by records, customertype, seqnum_t - seqnum_rt;
You can get the maximum just by using this as a subquery:
select customertype, max(num_zeros)
from (select customertype, count(*) as num_zeros
from (select t.*,
row_number() over (partition by records, customertype order by date) as seqnum_rt,
row_number() over (partition by customertype order by date) as seqnum_t
from t
) t
where records = 0
group by records, customertype, seqnum_t - seqnum_rt
) t
group by customertype;