MS SQL: Group by date [duplicate] - sql-server

Question

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Sql Date Grouping with avaliable dates in database
(3 answers)
Closed 4 years ago.
ID DateTime EmailCount
93 6/1/2014 00:00:00 4
94 6/2/2014 00:00:00 4
95 6/3/2014 00:00:00 2
96 6/4/2014 00:00:00 2
97 6/5/2014 00:00:00 2
98 6/6/2014 00:00:00 2
99 6/7/2014 00:00:00 2
73 6/8/2014 00:00:00 2
74 6/9/2014 00:00:00 2
75 6/10/2014 00:00:00 4
76 6/11/2014 00:00:00 4
77 6/12/2014 00:00:00 2
78 6/13/2014 00:00:00 2
79 6/14/2014 00:00:00 2
80 6/16/2014 00:00:00 2
81 6/17/2014 00:00:00 4
82 6/18/2014 00:00:00 4
83 6/19/2014 00:00:00 4
84 6/20/2014 00:00:00 4
100 6/21/2014 00:00:00 4
101 6/22/2014 00:00:00 4
102 6/23/2014 00:00:00 4
103 6/24/2014 00:00:00 4
89 6/27/2014 00:00:00 4
90 6/28/2014 00:00:00 4
91 6/29/2014 00:00:00 4
92 6/30/2014 00:00:00 4
104 7/1/2014 00:00:00 4
105 7/2/2014 00:00:00 4
106 7/3/2014 00:00:00 4
121 7/6/2014 00:00:00 2
122 7/7/2014 00:00:00 2
123 7/8/2014 00:00:00 2
Generated Output
Startdate EndDate EmailCount
6/3/2014 00:00:00 6/14/2014 00:00:00 2
6/16/2014 00:00:00 6/16/2014 00:00:00 2
7/6/2014 00:00:00 7/8/2014 00:00:00 2
6/1/2014 00:00:00 6/11/2014 00:00:00 4
6/17/2014 00:00:00 6/24/2014 00:00:00 4
6/27/2014 00:00:00 7/3/2014 00:00:00 4
Here, the generated output is not perfect because I want StartDate to EndDate in groups like: (6/3/2014 to 6/9/2014 and EmailCount = 2) and (6/10/2014 to 6/11/2014 and EmailCount =4) and (6/12/2014 to 6/14/2014 and EmailCount =2). Also, date not in database should not be added to group.

Answer 1

A somewhat complex query to explain, but here goes an attempt;
If the time is always midnight, you could use a common table expression to assign a row number to each row, and group by the difference between the date and row number. As long as the sequence is not broken (ie the dates are consecutive and with the same emailid) they will end up in the same group and an outer query can easily extract the start and end date for each group;
WITH cte AS (
SELECT dateandtime, emailid,
ROW_NUMBER() OVER (PARTITION BY emailid ORDER BY dateandtime) rn
FROM mytable
)
SELECT MIN(dateandtime) start_time,
MAX(dateandtime) end_time,
MAX(emailid) emailid
FROM cte GROUP BY DATEADD(d, -rn, dateandtime) ORDER BY start_time
An SQLfiddle to test with.
If the datetimes are not always midnight, the grouping will fail. If that's the case, you could add a common table expression that converts the datetime to a date as a separate step before running this query.

Answer 2

You're looking for runs of consecutive dates in blocks with the same EmailID. This assumes you have no gaps in the dates. I'm not sure it's the most elegant approach but you can find a lot of stuff on this topic.
with BlockStart as (
select t.StartDate, t.EmailID
from T as t left outer join T as t2
on t2.StartDate = t1.StartDate - 1 and t2.EmailID = t1.EmailID
where t2.StartDate is null
union all
select max(StartDate) + 1, null
from T
) as BlockStart
select
StartDate,
(select min(StartDate) - 1 from BlockStart as bs2 where bs2 > bs.StartDate) as EndDate,
EmailID
from BlockStart as bs
where
EmailID is not null
-- /* or */ exists (select 1 from BlockStart as bs3 where bs3.StartDate > bs.StartDate)