postgresql: splitting time period at event - postgresql

Question

I have a table of country-periods. In some cases, certain country attributes (e.g. the capital) changes on a date within a time period. Here I would like to split the country-period into two new periods, one before and one after this change.
Example:
Country | start_date | end_date | event_date
A | 1960-01-01 | 1999-12-31 | 1994-07-20
B | 1926-01-01 | 1995-12-31 | NULL
Desired output:
Country | start_date | end_date | event_date
A | 1960-01-01 | 1994-07-19 | 1994-07-20
A | 1994-07-20 | 1999-12-31 | 1994-07-20
B | 1926-01-01 | 1995-12-31 | NULL
I considered starting off with generate_series along these lines:
SELECT country, min(p1) as sdate1, max(p1) as sdate2,
min(p2) as sdate2, min(p2) as edate2
FROM
(SELECT country,
generate_series(start_date, (event_date-interval '1 day'), interval '1 day')::date as p1,
generate_series(event_date, end_date, interval '1 day')::date as p2
FROM table)t
GROUP BY country
But these seems way to inefficient and messy. Unfortunately I don't have any experience when it comes to writing functions. Any ideas on how I can solve this?

Answer 1

You can do UNION instead. This way you don't generate unnecessary rows
SELECT country, start_date,
CASE WHEN event_date BETWEEN start_date AND end_date
THEN event_date - 1
ELSE end_date
END AS end_date, event_date
FROM table1
UNION ALL
SELECT country, event_date, end_date, event_date
FROM table1
WHERE event_date BETWEEN start_date AND end_date
ORDER BY country, start_date, end_date, event_date
Here is a SQLFiddle demo
Output:
| country | start_date | end_date | event_date |
|---------|------------|------------|------------|
| A | 1960-01-01 | 1994-07-19 | 1994-07-20 |
| A | 1994-07-20 | 1999-12-31 | 1994-07-20 |
| B | 1926-01-01 | 1995-12-31 | (null) |