Break periods at the end of the month - sql

Question

SQL Server 2017
CREATE TABLE [TABLE_1]
(
PLAN_NR decimal(28,6) NULL,
START_DATE datetime NULL,
);
INSERT INTO TABLE_1 (PLAN_NR, START_DATE)
VALUES (1,'2020-05-01'), (2,'2020-08-01');
CREATE TABLE [TABLE_2]
(
PLAN_NR decimal(28,6) NULL,
PERIOD_NR decimal(28,6) NOT NULL
);
INSERT INTO TABLE_2 (PLAN_NR, PERIOD_NR)
VALUES (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8),
(2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8);
SQL-FIDDLE-LINK
In TABLE_1 there are plan number and plan start date.
TABLE_2 contains period numbers for each plan number.
I would like to compute the corresponding period start dates:
Each period is exactly 7 days long, unless the period contains a month end. Then the period should be divided into a range before the end of the month up to and including the last day of the month and a range after the end of the month.
The Select:
SELECT
t1.PLAN_NR, t2.PERIOD_NR,
FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM-dd') START_DATE
FROM
TABLE_1 t1
JOIN
TABLE_2 t2 ON t1.PLAN_NR = t2.PLAN_NR
ORDER BY
t1.PLAN_NR, t2.PERIOD_NR ASC
This returns the start data but without the extra to consider the respective month end:
+---------+-----------+------------+
| PLAN_NR | PERIOD_NR | START_DATE |
+---------+-----------+------------+
| 1 | 1 | 2020-05-01 |
| 1 | 2 | 2020-05-08 |
| 1 | 3 | 2020-05-15 |
| 1 | 4 | 2020-05-22 |
| 1 | 5 | 2020-05-29 |
| 1 | 6 | 2020-06-05 |
| 1 | 7 | 2020-06-12 |
| 1 | 8 | 2020-06-19 |
| 2 | 1 | 2020-08-05 |
| 2 | 2 | 2020-08-12 |
| 2 | 3 | 2020-08-19 |
| 2 | 4 | 2020-08-26 |
| 2 | 5 | 2020-09-01 |
| 2 | 6 | 2020-09-02 |
| 2 | 7 | 2020-09-09 |
| 2 | 8 | 2020-09-16 |
+---------+-----------+------------+
I would like an output like this:
+---------+-----------+----------------------+
| PLAN_NR | PERIOD_NR | START_DATE |
+---------+-----------+----------------------+
| 1 | 1 | 2020-05-01 |
| 1 | 2 | 2020-05-08 |
| 1 | 3 | 2020-05-15 |
| 1 | 4 | 2020-05-22 |
| 1 | 5 | 2020-05-29 |< --- period part before new month
| 1 | 6 | 2020-06-01 |< --- period part after new month
| 1 | 7 | 2020-06-05 |
| 1 | 8 | 2020-06-12 |
| 2 | 1 | 2020-08-05 |
| 2 | 2 | 2020-08-12 |
| 2 | 3 | 2020-08-19 |
| 2 | 4 | 2020-08-26 |< --- period part before new month
| 2 | 5 | 2020-09-01 |< --- period part after new month
| 2 | 6 | 2020-09-02 |
| 2 | 7 | 2020-09-09 |
| 2 | 8 | 2020-09-16 |
+---------+-----------+----------------------+

Answer 1

Use window functions (LEAD / LAG ) to get the start and end of the period ...
SELECT t1.PLAN_NR
, t2.PERIOD_NR
, FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM-dd') START_DATE
, CASE
WHEN
lead(
FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM-dd')
) over (partition by
FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM')
order by t2.period_nr)
IS NULL THEN '< --- period part before new month'
WHEN lag(
FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM-dd')
) over (partition by
FORMAT(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ),'yyyy-MM')
order by t2.period_nr)
IS NULL THEN '< --- period part after new month'
END as period_break
from TABLE_1 t1
join TABLE_2 t2
on t1.PLAN_NR = t2.PLAN_NR
order by t1.PLAN_NR, t2.PERIOD_NR asc
SQL Fiddle
PLAN_NR PERIOD_NR START_DATE period_break
1 1 2020-05-01 < --- period part after new month
1 2 2020-05-08 (null)
1 3 2020-05-15 (null)
1 4 2020-05-22 (null)
1 5 2020-05-29 < --- period part before new month
1 6 2020-06-05 < --- period part after new month
1 7 2020-06-12 (null)
1 8 2020-06-19 < --- period part before new month
2 1 2020-08-01 < --- period part after new month
2 2 2020-08-08 (null)
2 3 2020-08-15 (null)
2 4 2020-08-22 (null)
2 5 2020-08-29 < --- period part before new month
2 6 2020-09-05 < --- period part after new month
2 7 2020-09-12 (null)
2 8 2020-09-19 < --- period part before new month

Answer 2

SELECT
t1.PLAN_NR, t2.PERIOD_NR,
--row_number() over() but what if PERIOD_NR is not consecutive?
t2.PERIOD_NR + SUM(num.n) OVER(PARTITION BY t2.PLAN_NR ORDER BY t2.PERIOD_NR, num.n) AS PERIOD_NR_x,
FORMAT(CASE WHEN num.n = 1 THEN DATEADD(day, 1, EOMONTH(DATEADD (d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ))) ELSE DATEADD(d ,((t2.PERIOD_NR-1)*7) , t1.START_DATE ) END, 'yyyy-MM-dd') START_DATE
FROM
TABLE_1 t1
JOIN
TABLE_2 t2 ON t1.PLAN_NR = t2.PLAN_NR
CROSS APPLY
(
SELECT 0 AS n
UNION ALL
--new row for month change
SELECT 1 AS n
WHERE DATEDIFF(month, DATEADD(d ,(t2.PERIOD_NR-1)*7 , t1.START_DATE), DATEADD(d ,t2.PERIOD_NR*7 , t1.START_DATE)) = 1
) as num
ORDER BY
t1.PLAN_NR, t2.PERIOD_NR ASC