Need a simple query to calculate sequence length in SQL Server - sql

Question

I have this view that represent the status of connections for each user to a system inside table as below:
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|id | date | User | Connexion |
|1 | 01/01/2018 | A | 1 |
|2 | 02/01/2018 | A | 0 |
|3 | 03/01/2018 | A | 1 |
|4 | 04/01/2018 | A | 1 |
|5 | 05/01/2018 | A | 0 |
|6 | 06/01/2018 | A | 0 |
|7 | 07/01/2018 | A | 0 |
|8 | 08/01/2018 | A | 1 |
|9 | 09/01/2018 | A | 1 |
|10 | 10/01/2018 | A | 1 |
|11 | 11/01/2018 | A | 1 |
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The target output would be to get the count of succeeded and failed connection order by date so the output would be like that
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|StartDate EndDate User Connexion Length|
|01/01/2018 | 01/01/2018 | A | 1 | 1 |
|02/01/2018 | 02/01/2018 | A | 0 | 1 |
|03/01/2018 | 04/01/2018 | A | 1 | 2 |
|05/01/2018 | 07/01/2018 | A | 0 | 3 |
|08/01/2018 | 11/01/2018 | A | 1 | 4 |
---------------------------------------------------------------

Answer 1

This is what is called a gaps-and-islands problem. The best solution for your version is a difference of row numbers:
select user, min(date), max(date), connexion, count(*) as length
from (select t.*,
row_number() over (partition by user order by date) as seqnum,
row_number() over (partition by user, connexion order by date) as seqnum_uc
from t
) t
group by user, connexion, (seqnum - seqnum_uc);
Why this works is a little tricky to explain. Generally, I find that if you stare at the results of the subquery, you'll see how the difference is constant for the groups that you care about.
Note: You should not use user or date for the names of columns. These are keywords in SQL (of one type or another). If you do use them, you have to clutter up your SQL with escape characters, which just makes the code harder to write, read, and debug.