SQL Server 2019

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create table SampleData (Id int, ContactId int, Item varchar(25),CreatedOn date)

insert into SampleData
VALUES(100,2500,'Some item name 1212', '9/5/2020'),
      (104,2500,'Some item name 2232', '9/15/2020'),
      (109,2500,'Some item name 3434', '9/20/2020'),

      (112,3000,'Some item name 5422', '8/1/2020'),
      (132,3000,'Some item name 344', '9/5/2020'),
      (134,3000,'Some item name 454', '9/15/2020'),

      (139,3500,'Some item name 6455', '7/5/2020'),
      (146,3500,'Some item name 546', '8/5/2020'),
      (142,3500,'Some item name 867', '9/5/2020'),
      (149,3500,'Some item name 677', '9/15/2020'),
      (150,3500,'Some item name 888', '9/19/2020')
11 rows affected

select sd.*,
       sum(case when prev_createdon > dateadd(day, -28, createdon) then 0 else 1 end) over
           (partition by contactid order by createdon) as grouping
from (select sd.*,
             lag(createdon) over (partition by contactid order by createdon) as prev_createdon
      from SampleData sd
     ) sd


Id
ContactId
Item
CreatedOn
prev_createdon
grouping


100
2500
Some item name 1212
2020-09-05
null
1

104
2500
Some item name 2232
2020-09-15
2020-09-05
1

109
2500
Some item name 3434
2020-09-20
2020-09-15
1

112
3000
Some item name 5422
2020-08-01
null
1

132
3000
Some item name 344
2020-09-05
2020-08-01
2

134
3000
Some item name 454
2020-09-15
2020-09-05
2

139
3500
Some item name 6455
2020-07-05
null
1

146
3500
Some item name 546
2020-08-05
2020-07-05
2

142
3500
Some item name 867
2020-09-05
2020-08-05
3

149
3500
Some item name 677
2020-09-15
2020-09-05
3

150
3500
Some item name 888
2020-09-19
2020-09-15
3


select contactid, min(createdon), max(createdon), min(id),
       max(case when seqnum = 1 then item end) as item
from (select sd.*,
             row_number() over (partition by contactid, grouping order by createdon) as seqnum
      from (select sd.*,
                   sum(case when prev_createdon > dateadd(day, -28, createdon) then 0 else 1 end) over
                        (partition by contactid order by createdon) as grouping
            from (select sd.*,
                         lag(createdon) over (partition by contactid order by createdon) as prev_createdon
                  from SampleData sd
                 ) sd
           ) sd
     ) sd
group by contactid, grouping
     


contactid
(No column name)
(No column name)
(No column name)
item


2500
2020-09-05
2020-09-20
100
Some item name 1212

3000
2020-08-01
2020-08-01
112
Some item name 5422

3500
2020-07-05
2020-07-05
139
Some item name 6455

3000
2020-09-05
2020-09-15
132
Some item name 344

3500
2020-08-05
2020-08-05
146
Some item name 546

3500
2020-09-05
2020-09-19
142
Some item name 867


Warning: Null value is eliminated by an aggregate or other SET operation.


select sd.*
from (select sd.*,
             lag(createdon) over (partition by contactid order by createdon) as prev_createdon
      from SampleData sd
     ) sd
where prev_createdon < dateadd(day, -28, createdon) or prev_createdon is null


Id
ContactId
Item
CreatedOn
prev_createdon


100
2500
Some item name 1212
2020-09-05
null

112
3000
Some item name 5422
2020-08-01
null

132
3000
Some item name 344
2020-09-05
2020-08-01

139
3500
Some item name 6455
2020-07-05
null

146
3500
Some item name 546
2020-08-05
2020-07-05

142
3500
Some item name 867
2020-09-05
2020-08-05

Id	ContactId	Item	CreatedOn	prev_createdon	grouping
100	2500	Some item name 1212	2020-09-05	null	1
104	2500	Some item name 2232	2020-09-15	2020-09-05	1
109	2500	Some item name 3434	2020-09-20	2020-09-15	1
112	3000	Some item name 5422	2020-08-01	null	1
132	3000	Some item name 344	2020-09-05	2020-08-01	2
134	3000	Some item name 454	2020-09-15	2020-09-05	2
139	3500	Some item name 6455	2020-07-05	null	1
146	3500	Some item name 546	2020-08-05	2020-07-05	2
142	3500	Some item name 867	2020-09-05	2020-08-05	3
149	3500	Some item name 677	2020-09-15	2020-09-05	3
150	3500	Some item name 888	2020-09-19	2020-09-15	3