Overlapping effective dates aggregation - sql

Question

I am trying to aggregate overlapping effective dates. Any gaps between dates should be considered as separate rows. I am using min and max and I am getting below output but would like to see expected output.
My query
WITH test_data AS (
SELECT '2020-01-01' AS date_from,
'2020-01-03' AS date_to,
'1' AS product
UNION ALL
SELECT '2020-01-05' AS date_from,
'2020-01-07' AS date_to,
'1' AS product
UNION ALL
SELECT '2020-01-06' AS date_from,
'2020-01-10' AS date_to,
'1' AS product
)
SELECT product,
MIN(date_from) AS date_from,
MAX(date_to) AS date_to
FROM test_data
GROUP BY 1;
Source data
date_from
date_to
product
2020-01-01
2020-01-03
1
2020-01-05
2020-01-07
1
2020-01-06
2020-01-10
1
Output table
date_from
date_to
product
2020-01-01
2020-01-10
1
Expected output
date_from
date_to
product
2020-01-01
2020-01-03
1
2020-01-05
2020-01-10
1
Thanks in advance !

Answer 1

This is a type of gaps-and-islands problem. I recommend an approach like this:
SELECT product,
MIN(date_from) AS date_from,
MAX(date_to) AS date_to
FROM (SELECT td.*,
SUM(CASE WHEN prev_date_to >= date_from THEN 0 ELSE 1 END) OVER (PARTITION BY product ORDER BY date_to) as grp
FROM (SELECT td.*,
MAX(date_to) OVER (PARTITION BY product ORDER BY date_from ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) as prev_date_to
FROM test_data td
) td
) td
GROUP BY grp, product
ORDER BY product, MIN(date_from);
Here is a db<>fiddle.
What is this doing? The innermost subquery is getting the latest date_to on previous rows. This is used to determine if each row is "connected" to the previous row or if it starts a new grouping.
The middle subquery has logic which is a cumulative sum of when the rows start a new group. The outer query then aggregates by this grouping.

Answer 2

Merging of date ranges could be achieved with MATCH_RECOGNIZE.
Data preparation:
CREATE OR REPLACE TABLE test_data AS
SELECT '2020-01-01'::DATE AS date_from, '2020-01-03'::DATE AS date_to, '1' AS product
UNION ALL
SELECT '2020-01-05'::DATE AS date_from, '2020-01-07'::DATE AS date_to, '1' AS product
UNION ALL
SELECT '2020-01-06'::DATE AS date_from, '2020-01-10'::DATE AS date_to, '1' AS product;
Query:
SELECT *
FROM test_data t
MATCH_RECOGNIZE(
PARTITION BY product
ORDER BY date_from, date_to
MEASURES FIRST(date_from) date_from, MAX(date_to) date_to
PATTERN(a* b)
DEFINE a AS MAX(date_to) OVER() >= NEXT(date_from)
) mr;
db<>fiddle demo - Oracle
Related reading: Merging Overlapping Date Ranges with MATCH_RECOGNIZE by stewashton