Join grouping by periods, full join not working as intended - sql

Question

I have a sales table:
SALES
|---------|-------------|-------------|
| order | ammount | date |
|---------|-------------|-------------|
| 001 | $2,000 | 2018-01-01 |
| 002 | $3,000 | 2018-01-01 |
| 003 | $1,500 | 2018-01-03 |
| 004 | $1,700 | 2018-01-04 |
| 005 | $1,800 | 2018-01-09 |
| 006 | $4,200 | 2018-01-11 |
|---------|-------------|-------------|
Aditionally, I have a table that groups said sales according to arbitrary time periods:
BUDGET PERIODS
|---------|-------------|--------------|
| ID | start_date | end_date |
|---------|-------------|--------------|
| 1 | 2018-01-01 | 2018-01-02 | <- notice this is a 2 day period...
| 2 | 2018-01-03 | 2018-01-05 | <-- but this is 3 days
|---------|-------------|--------------|
So, my result table query looked like this:
GROUPED SALES
|--------------|---------------|-----------------|
| start_date | end_date | ammount |
|--------------|---------------|-----------------|
| 2018-01-01 | 2018-01-02 | $5,000 |
| 2018-01-03 | 2018-01-05 | $3,200 |
|--------------|---------------|-----------------|
I accomplished it by a query as such:
SELECT
bp.start_date,
bp.end_date,
SUM(s.ammount)
FROM
budget_periods bp
LEFT JOIN
sales s ON s.date >= bp.start_date AND s.date <= bp.end_date
GROUP BY
start_date,
end_date
Everything is awesome, then. BUT, I notice that, of course, some sales are not included because they're not in budget periods. Hence, I want to include them "somewhere". I decided that that "somewhere" would be the week of the sale (using the week truncate function in Postgres). Hence, my grouped sales should look like this now:
GROUPED SALES
|--------------|---------------|-----------------|
| start_date | end_date | ammount |
|--------------|---------------|-----------------|
| 2018-01-01 | 2018-01-02 | $5,000 |
| 2018-01-03 | 2018-01-05 | $3,200 |
| 2018-01-08 | 2018-01-14 | $6,000 |
|--------------|---------------|-----------------|
Notice that if you truncate-to-week both 2018-01-09 and 2018-01-11, it shows 2018-01-08. To calculate my end_date, the budget period is "defaulted" to seven days, so it's six days later than the start_date.
So, I modified the query into a FULL JOIN like this:
SELECT
COALESCE(bp.start_date, DATE_TRUNC('WEEK', s.date)) AS new_start_date,
COALESCE(bp.end_date, DATE_TRUNC('WEEK', s.date) + INTERVAL '6 DAY') AS new_end_date,
SUM(s.ammount)
FROM
budget_periods bp
FULL JOIN
sales s ON s.date >= bp.start_date AND s.date <= bp.end_date
GROUP BY
new_start_date,
new_end_date
But then, the result table is the same as when I had a LEFT JOIN. How should I approach this?
Thank you for your time in reading such a long time to explain issue.

Answer 1

If you want all rows in sales then make that the first table in a LEFT JOIN. However, I think the FULL JOIN should work, as should this LEFT JOIN:
SELECT COALESCE(bp.start_date, DATE_TRUNC('WEEK', s.date)) as new_start_date,
COALESCE(bp.end_date, DATE_TRUNC('WEEK', s.date) + interval '6 day') as new_end_date,
SUM(s.amount)
FROM sales s LEFT JOIN
budget_periods bp
ON s.date >= bp.start_date AND s.date <= bp.end_date
GROUP BY new_start_date, new_end_date;
The only reason things would be filtered out of the FULL JOIN is through a WHERE clause, but you don't have one.